The Appendages, Anatomy, and Relationships of Trilobites Part 9

=Ceraurus pleurexanthemus= Green.

(pl. 11; text figs. 12, 17-19, 21, 22, 24, 29, 30.)

Ill.u.s.trated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875, pl. 11;--31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl.

1, fig. 3;--Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl.

4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol.

67, 1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12 (not _Calymene_), (not 15, _Calymene_); pl. 28, figs. 1-5; pl. 34, fig. 1; pl. 35, fig. 7.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.--Bernard, The Apodidae, 1892, text figs. 46, 51.

_Cephalic Appendages._

No trace of antennules has yet been found.

I find only three sections cut through the plane of the hypostoma of Ceraurus which show anything of the cephalic appendages, and no one of them is very satisfactory. The best is No. 22, the one figured by Walcott (pl. 3, fig. 2, 1881; pl. 27, fig. 12, 1918), but one should remember that this section is not actually cut in the plane of the hypostoma but is a slice diagonally through the head, cutting through one eye and the posterior end of the hypostoma. It shows what seem to be the c.o.xopodites of the second, third, and fourth pairs of cephalic appendages, the exopodites of the third and fourth pairs, and the metastoma. If this interpretation is correct, the first pair of gnathites lay alongside the hypostoma or under its edge, and were feebly developed, the second pair were attached in front of the tip of the hypostoma, curved back close to it, and their inner ends reached the sides of the metastoma. The third and fourth pairs were back of the metastoma, the third pair was stronger than the second, and the fourth probably like the third.

[Ill.u.s.tration: Fig. 17. Transverse section of _Ceraurus pleurexanthemus_, showing the relation of the c.o.xopodite to the appendifer. Traced from a photographic enlargement of the slice.

Specimen 128. 4/5.]

[Ill.u.s.tration: Fig. 18. Slice of _Ceraurus pleurexanthemus_, showing a nearly continuous section of an endopodite and an exopodite above it.

The latter is so cut as to show only the edge of the shaft and the bases of a few setae. Traced from a photographic enlargement. Specimen in. 4.]

Specimen 92 shows traces of the slender endopodites belonging to the cephalon, but no details. Specimen 22 shows on one side exopodites (epipodites of Walcott) belonging to the third and fourth cephalic appendages. That belonging to the third shows some long setae and a trace of the shaft, while the one on the fourth appendage (third c.o.xopodite) has a portion of a broad shaft and a number of long setae.

It should again be remembered that the slice does not cut through the plane of the exopodite, but across it at a low angle, so that a part but not all of the shaft is shown. On the other side of this slice there is a fairly good section of one of the thoracic exopodites. It is, however, turned around in the opposite direction from the others, as would be expected in an enrolled specimen.

Specimens 4 and 5 (pl. 1, figs. 4, 5, 1881) are slices cut diagonally through the head of Ceraurus, in front of the posterior tip of the hypostoma. They show fragments of endopodites and exopodites which may be interpreted as practically identical in form with those of the thorax. Due to the diagonal plane in which the section is cut, slice 5 shows the c.o.xopodites of two pairs of appendages, one lying nearer the median cavity than the other. It is extremely difficult to visualize the interpretation of such sections.

_Thoracic Appendages._

A transverse section through a thoracic segment (No. 128, our fig. 17) shows the relation of c.o.xopodite to appendifer to be the same as in _Calymene_, the upper side of the c.o.xopodite having a notch a little outward from the middle. After seeing that specimen, it is possible to understand slice No. 168, which shows longitudinal sections through a number of c.o.xopodites of the thorax, with fragments of both exopodites and endopodites articulated at the distal ends. These and longitudinal vertical sections like No. 18 (pl. 2, fig. 8, 1881) show that the endobases taper inward, and the general uniformity in width in sections taken at various angles indicates that the c.o.xopodites were not greatly flattened.

A unique slice (No. 111, pl. 2, fig. 2, 1881; pl. 27, fig. 1, 1918; our fig. 18) shows a nearly complete thoracic endopodite, and above it a part of the proximal end of the exopodite of the same segment. When one considers that out of over two thousand sections only this one shows the six successive segments of an endopodite, one realizes how futile it is to expect that dozens of the equally slender "spirals"

should be cut so as to show practically all their turns.

This endopodite is slender, all the segments have nearly the same length and diameter, though there is a slight taper outward, each segment is expanded distally for the articulation of the next, and there are small spines on the distal ends of some of them. There is probably a terminal spine present, though it is neither so long nor so plainly visible as in Walcott's photograph.

The exopodite on this same specimen was evidently cut diagonally across near the setiferous edge, showing a section through the shaft and the bases of seven setae (fig. 18). This section is so exactly what would be obtained by cutting similarly an exopodite of either Neolenus or _Triarthrus_ that it should in itself dispose of the "spiral-exopodite" theory.

Several sections have already been ill.u.s.trated showing sections across the setae of the exopodites (pl. 3, figs. 4-6, 1881; pl. 27, figs. 3, 4, 9, 1918), and similar sections are not uncommon. Only a very few, however, show sections in the plane of the exopodite. If only No. 111, described above, were known, it would be inferred that the exopodite had a slender shaft as in _Calymene_, but another good slice, No. 80 (fig. 12, ante) shows that the blade was rather broad, though not so broad as in Neolenus. The other specimen is No. 22, which has already been discussed. The thoracic exopodite of this specimen has been very incorrectly figured by Walcott, as it shows no such palmate shaft as he has indicated, but a long blade-like one is outlined, though its entire width is not actually shown.

_Pygidial Appendages._

Sections 14 and 18 (pl. 2, figs. 4, 8, 1881) prove the presence under the pygidium of three pairs of appendages, the c.o.xopodites and fragments of endopodites of which are shown. Nothing is known of the exopodites.

_Relation of Hypostoma to Cephalon._

In Ceraurus the body portion and posterior end of the hypostoma are roughly oval, about as wide as the glabella at its broadest part, and the posterior edge extends back to within 0.5 to 1 mm. of the neck furrow. The posterior pair of appendifers are behind the hypostoma, while the second pair are in front of its posterior end but escape being covered by it on account of its oval shape. At the anterior end the hypostoma is widened by the presence of two side lappets which extend beyond the boundaries of the glabella. In both Ceraurus and Cheirurus the anterior edge of the hypostoma fits against the doublure at the anterior margin of the head and the epistoma is either entirely absent or is so narrow as not to be seen in specimens in the ordinary state of preservation. A section across the cephalon of _Ceraurus pleurexanthemus_ at the horizon of the eyes shows the sides of the hypostoma fitting closely against the sides of the glabella (Walcott's pl. 1, fig. 1). Further back on the head it is not in contact with the dorsal test, and the gnathobases extend beneath it.

Restoration of _Ceraurus pleurexanthemus_. (pl. 11; text fig. 19.)

The restoration of the appendages of _Ceraurus pleurexanthemus_ is a tentative one, based upon a careful study of the translucent sections prepared by Doctor Walcott. In no case among these sections is the actual test of any appendage preserved, and the real form of each part is generally obscured by the crystallization of the calcite which fills the s.p.a.ces formerly occupied by animal matter.

[Ill.u.s.tration: Fig. 19. Restoration of a transverse section of the thorax of _Ceraurus pleurexanthemus_ Green, showing the relation of the appendages to the appendifers and the ventral membrane. The probable positions of the heart and alimentary ca.n.a.l are indicated.]

No section shows anything which can be identified as any part of the antennules, so that these organs have been supplied from a.n.a.logy with _Triarthrus_.

There are undoubtedly four pairs of biramous Cephalic appendages, but their points of attachment are not so obvious. There are two pairs of conspicuous appendifers on the posterior part of the cephalon and another pair almost concealed by the hypostoma. It is probable that the appendages of the cephalon were not attached directly beneath them, as the four pairs have to be placed within the s.p.a.ce occupied by the three pairs of appendifers. As the mouth is in front of the posterior end of the hypostoma, the gnathites of the first pair of biramous appendages may have extended beneath that organ, or they may have lain beside it, and only become functional when the hypostoma was dropped down in the feeding position. The second pair of gnathites reached just to the tip of the hypostoma, and the other two pairs seemingly curved backward behind it.

The points of attachment on the thorax, as shown clearly in sections, were directly beneath the lower ends of the appendifers. The endopodites were long enough to reach to or a little beyond the outer extremities of the pleural spines, while the exopodites were apparently somewhat shorter. Each endopodite consisted of six short, fairly stout segments, each with at least two spines on the somewhat expanded distal ends. The exact form of the exopodites could not be made out. The shaft was apparently rather short, unsegmented, and fairly broad. The setae appear from the sections to have been more or less blade-shaped and to have overlapped, as do those of the exopodites of _Cryptolithus_. Judging from their position in the sections, the setae not only bordered the posterior side of the shaft, but radiated out from the end as well.

The pygidium shows three pairs of functional appendifers, hence three pairs of appendages have been supplied. There is a fourth pair of rudimentary appendifers, but as they are beneath the doublure they could not have borne ambulatory appendages.

The Appendages of Acidaspis trentonensis Walcott.

(pl. 6, fig. 6.)

A single individual of _Acidaspis trentonensis_, obtained from the same locality and horizon as the specimens of _Triarthrus_ and _Cryptolithus_, when cleaned from the ventral side shows a number of poorly preserved endopodites which seem very similar in shape and position to those of _Triarthrus_. One endopodite on the right side of the head and the first five on the right side of the thorax are the best shown. All are slender, are directed first forward at an angle of about 45 with the axis, then, except in the case of the cephalic appendage, turn backward on a gentle curve and extend a little distance beyond the margin of the test, but not as far as the tips of the lateral spines of the thoracic segments.

The individual segments of the endopodites can not be seen clearly enough to make any measurements. On the fourth and fifth endopodites of the thorax, some of the segments seem to be broad and triangular as in _Triarthrus_. All that can be seen indicates that _Acidaspis_ had appendages entirely similar to those of _Triarthrus_, but perhaps not quite so long, as they seem not to have projected beyond the limits of the lateral spines. There are no traces of antennules nor, unfortunately, of exopodites.

_Measurements:_ Length 8 mm.

Walcott (1881, p. 206) stated that his sections had shown the presence in this species of legs "both cephalic and thoracic" and also the "spiral branchiae." His specimens were from the Trenton at Trenton Falls, New York.

The Appendages of Cryptolithus.

=Cryptolithus tessellatus= Green.

(pl. 6, fig. 7; pls. 7-9; text figs. 20, 25, 45, 46.)

(See also Part IV.)

Ill.u.s.trated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3.

When Professor Beecher wrote his short article on the "Structure and Appendages of _Trinucleus_" (1895), he had only three specimens showing appendages. In his later work he cleaned several more, so that there are now thirteen specimens of _Trinucleus_ = _Cryptolithus_ available for study, though some of these do not show much detail. In his last and unpublished study, Beecher devoted the major part of his attention to this genus, and summarized his findings in the drawings which he himself made of the best individuals (text figs. 45, 46).

Valiant (1901) stated that he had found a _Trinucleus_ with antennae in the Frankfort shale south of Rome, New York. The specimen has not been figured.

None of the specimens shows much more of the appendages of the cephalon than, the hypostoma and the antennules, so that we are still in ignorance about the mouth parts.

The most striking characteristics of the appendages are as follows: the antennules are long, and turn backward instead of forward; none of the limbs projects beyond the margin of the dorsal test; the exopodites extend beyond the endopodites, reaching very nearly to the margin of the test; the endopodites are not stretched out at right angles to the axis, but the first three segments have a forward and outward direction as in _Triarthrus_, while the last four turn back abruptly so that they are parallel to the axis; the limbs at the anterior end of the thorax are much more powerful than the others; the dactylopodites of the endopodites show a fringe of setae instead of three spines as in _Triarthrus_ and _Neolenus_. All these would, as Beecher has already suggested, seem to be adaptations to a burrowing habit of life, the antennules being turned backward and the other appendages kept within the shelter of the dorsal test in order to protect them, and the anterior endopodites enlarged and equipped with extra spines to make them more efficient digging and pushing organs.

_Restoration of Cryptolithus._

(Text fig. 20.)

It should be definitely understood that the present figure is a restoration and not a drawing of a specimen, and that there are many points in the morphology of _Cryptolithus_ about which no information is available, especially about the appendages under the central portion of the cephalon. The information afforded by all the figures published in this memoir is combined here. As gnathites are preserved on none of the specimens, those represented in the figure are purely conventional.