The Appendages, Anatomy, and Relationships of Trilobites Part 4

The club-shaped bodies lying within the axis are the gnathobases attached at the sides of the axis; the curved members extending outward from the gnathobases are the endopodites; the longitudinal ridges in the ventral membrane between the inner ends of the gnathobases are the b.u.t.tresses and apodemes of the mesosternites; the slender oblique rod-like bodies shown in the right pleural region in Walcott's figure are portions of the fringes of the exopodites.

In 1910, Mr. W. C. King of Ottawa, Ontario, found at Britannia, a few miles west of Ottawa, the impression in sandstone of the under surface of a large specimen of _Isotelus arenicola_, described on a later page (p. 39).

Finally (1918, p. 133, pl. 24, figs. 3, 3a; pl. 25), Walcott has redescribed the specimen from Ohio, presenting a new and partially restored figure. He refers also to the specimen from Ottawa under the name _Isotelus covingtonensis?_ Foerste (not Ulrich). He advances the view, which I am unable to share, that the cylindrical appearance of the segments of the appendages of _Isotelus_ is due to post-mortem changes.

=Isotelus latus= Raymond.

(pl. 10, fig. 1.)

Ill.u.s.trated: _Asaphus platycephalus_ Billings, Quart. Jour. Geol.

Soc., London, vol. 26, 1870, pl. 31, figs. 1-3; pl. 32, figs. 1, 2.--Woodward, Geol. Mag., vol. 8, 1871, pl. 8, figs. 1, 1a.--Gerstacker, in Bronn's "Kla.s.sen u. Ordnungen d. Thier-Reichs,"

1879, pl. 49, fig. 1.--von Koenen, N. Jahrb. f. Min., etc., vol. 1, 1880, pl. 8, fig. 8.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser.

6, vol. 12, 1881, pl. 12, fig. 45.

_Isotelus latus_ Raymond, Bull. Victoria Mem. Mus., Geol. Survey Canada, No. 1, 1913, p. 45 (species named).

_Isotelus covingtonensis?_ Walcott (not Foerste), Smithson. Misc.

Coll., vol. 67, 1918, p. 134.

Knowledge of the appendages of this species is derived from the specimen which Billings described in 1870. It was found in the Trenton, probably the Middle Trenton, near Ottawa, Ontario, and is preserved in the Victoria Memorial Museum at Ottawa.

Viewed from the upper surface, it shows a large part of the test, but is broken along the sides, so that parts of the free cheeks, considerable of the pleural lobes of the thorax, and one side of the pygidium are missing. Viewed from the lower surface, the appendages are practically confined to the cephalon and thorax.

A short time before his death, Professor Beecher had this specimen and succeeded in cleaning away a part of the matrix so that the appendages show somewhat more clearly than in Billings' time, but they are not so well preserved as on the Mickleborough specimen, found in Ohio somewhat later.

The hypostoma is in place and well preserved; the posterior points are but 3 mm. in advance of the posterior margin of the cephalon. Behind the hypostoma there are only two pairs of cephalic appendages, the first of which is represented by the c.o.xopodite and a trace of the endopodite. The outer end of the c.o.xopodite is close to the outer margin of one of the p.r.o.ngs of the hypostoma and about 3 mm. in front of its posterior end. The gnathobase curves backward and inward, and appears to pa.s.s under the tip of the hypostoma. There were probably two appendages in front of this, whose gnathobases projected under the hypostoma, but the specimen shows nothing of them unless it be that one small fragment about 2 mm. back of the center is really a part of a gnathobase.

The specimen retains only the c.o.xopodite and basipodite of the posterior cephalic appendage on the left side. The c.o.xopodite is long and apparently cylindrical, the cross-section being of uniform diameter throughout the length. The inner portion is nearly straight, while the outer part is curved gently forward.

It is possible to make out remains of eight pairs of appendages on the thorax, some of them represented by c.o.xopodites only, but most with more or less poorly preserved endopodites as well. No exopodites are visible. The c.o.xopodites of the thorax seem to be of the same form as the last one on the cephalon, but slightly less curved. All are long and heavy, and there seems to be no decrease in size toward the pygidium. The endopodites are very imperfectly shown. They seem to be longer than those of _Isotelus maximus_, and the segments, while of less diameter than the c.o.xopodites, do not show so great a contrast to them as do those of that species. The direction of the endopodites is diagonally forward, and the outer portions do not appear to be curved backward as in _Isotelus maximus_. It would appear also that the endopodites were nearly or quite long enough to reach the outer margin of the dorsal test. On no endopodite can more than three segments be definitely distinguished, but the longest ones are the most obscurely segmented.

No appendages are preserved on the pygidium, but at one side of the median groove there are two projections which may be processes to which the appendages were attached.

_Measurements:_ Total length of specimen, 109 mm. Probable length when complete, 116 mm. Length of cephalon, 40 mm.; width at genal angles, restored, about 62 mm. (Billings' restoration). Width of doublure of front of cephalon on median line, 17 mm.; length of hypostoma, 20 mm.

Length of c.o.xopodite of last appendage on left side of cephalon, 10.5 mm.; length of basipodite of the same appendage, 5 mm. Diameter of c.o.xopodite, 2 mm.; diameter of basipodite, 1.5 mm. Length of c.o.xopodite on left side of the second segment of the thorax, 11 mm.; diameter, about 2.5 mm. Length of basipodite of the same, 5 mm.; diameter, about 1.5 mm. Length of ischiopodite, 3.5 mm.; diameter, about 1.5 mm. Length of meropodite, 2.5 mm. (this may be less than the total length as the segment is not completely exposed.) Distance between proximal ends of gnathobases of the fifth thoracic segment, about 7 mm. Distance between outer ends of the c.o.xopodites of the first thoracic segment (estimated from measurements on the left side), 27 mm Distance apart of the dorsal furrows at the first thoracic segment, 27 mm. Length of the longest exopodite which can be traced, about 20 mm.

=Isotelus maximus= Locke.

(pl. 10, fig. 2.)

Ill.u.s.trated: Mickleborough, Jour. Cincinnati Soc. Nat. Hist., vol.

6, 1883, p. 200, figs. 1-3 (endopodites and c.o.xopodites). Walcott, Science, vol. 3, 1884, p. 279, fig. 1 (endopodites, c.o.xopodites, and traces of exopodites). Woodward, Geol. Mag., dec. 3, vol. 1, 1884, p. 162, figs. 1-3 (copies of Mickleborough's figures).

Bernard, The Apodidae, 1892, text fig. 49. Beecher, Amer. Jour.

Sci., vol. 13, 1902, p. 169, pl. 5. figs. 5, 6 (outline from one of Mickleborough's figures and an original figure). Walcott, Smithson.

Misc. Coll., vol. 67, 1918, p. 133, pl. 24, figs. 3, 3a; pl. 25, fig. 1.

This specimen, which conies from the Richmond strata 2 miles north of Oxford, Ohio, is the best preserved of the specimens of _Isotelus_ with appendages which has so far been found. The individual consists of two parts, the actual specimen, and the impression of the ventral side.

To describe it I am using very skillfully made plaster reproductions of both parts, presented to the Museum of Comparative Zoology by Doctor Charles D. Walcott, and presumably made after he cleaned the specimen as described in Science (1884). I have also an enlarged photograph (pl. 10, fig. 2) which seems to have been made after some later period of cleaning, probably by Professor Beecher, and I have examined the original specimens in Washington.

Viewed from the dorsal side, it is seen that the individual is very imperfect, the greater part of the cephalon being removed by a diagonal break which cuts off the anterior third of the left eye and extends to the front of the second thoracic segment on the right side.

The ends of the pleura of both sides of the thorax are broken away, as are also the greater parts of the pleural lobes and the posterior end of the pygidium. On the ventral side, merely the posterior tips of the hypostoma remain, but the distal ends of the appendages were so far within the outer margin that the appendagiferous area is quite fully retained.

The most conspicuous feature of this specimen is the presence of nine pairs of large c.o.xopodites behind the hypostoma, and of the remains of ten pairs of endopodites, making in all ten pairs of appendages which are easily seen. The apportionment of these segments to cephalon, thorax, and pygidium is not agreed upon by the people who have examined the specimens, but if one remembers that it is the outer and not the inner end of the c.o.xopodite which articulates with the appendifer, it at once becomes evident that the first two pairs of appendages on the specimen are the last two pairs belonging to the cephalon, and that the next eight pairs are those of the thorax.

The impressions of fourteen pairs of c.o.xopodites are readily counted on the pygidium, and as Doctor Walcott noted sixteen pairs on the actual specimens, his number was probably correct.

_Cephalon._

Projecting the line of the back of the cephalon through from the dorsal side, it is found that the posterior tips of the hypostoma are 7 mm. in front of the posterior margin of the cephalon, and that the points of attachment of the posterior pair of cephalic appendages (the second pair shown on the specimen) are just within the posterior margin. The gnathobases of this pair of appendages extend back some distance beneath the thorax, and so give the impression that they belong to that part of the body. So far as can be determined, the cephalic appendages do not differ in any way from those of the thorax.

On the mould of the ventral surface, just outside of the lateral edge of the right lobe of the hypostoma, is the somewhat imperfectly shown impression of the endopodite of the third cephalic appendage. The point of junction of the endopodite and c.o.xopodite is about 2 mm. in front of the tip of the adjacent branch of the hypostoma, and the gnathobase is curved around just behind it. This accounts for three of the pairs of cephalic appendages. The second cephalic appendages must have thrust their gnathobases under the p.r.o.ngs of the hypostoma, and the endopodites were probably close to its edge. No trace of this pair appears on the specimen.

_Thorax._

The thoracic appendages are the best preserved of any, and show the large c.o.xopodites and the more slender endopodites which do not extend to the outer margin of the test. The latter extend forward and outward for about one half their length, then turn backward in a graceful curve.

Walcott's figure in Science shows hair-like markings on the under side of the right half of the thorax. These were interpreted by both Walcott and Beecher as fringes of the exopodites, but since the setae of those organs on all other trilobites are always above the endopodites, while these are represented as below them, it would seem doubtful if this interpretation can be sustained. Furthermore, I find no trace of them on either cast or mould, and the actual specimen does not now show them.

_Pygidium._

The c.o.xopodites and endopodites of the pygidium seem to be similar to those on the thorax, but both are shorter and more slender, and the former decrease in length rapidly toward the posterior end. As mentioned above, it is not perfectly plain how many appendages are present, but I have accepted Doctor Walcott's count of sixteen pairs.

Of the endopodites only the barest traces are seen, and of exopodites nothing.

One point of considerable interest in this specimen is the thickness, as it probably gives some measure of the s.p.a.ce occupied by the animal.

In _Triarthrus_ and other trilobites from Rome, New York, the appendages are pressed directly against the dorsal test, but in this specimen a considerable s.p.a.ce intervenes between the plane of the appendages and the sh.e.l.l. Between the central furrow and the inner surface of the dorsal test at the anterior end of the thorax is a distance of 13 mm. and under the dorsal furrows the thickness is about 7 or 8 mm., no accurate measurement being possible in the present state of the specimen.

_Measurements:_ Length of specimen on median line, 121 mm.; probable original length, about 195 mm. (Walcott's restoration). Length of thorax, 58 mm.[1] Width of axial lobe at the first thoracic segment, 45 mm.; total width as preserved, 92 mm.; width as estimated from the mould of the ventral surface, no mm.; Walcott's restoration, 105 mm.

[Footnote 1: If this specimen had the same proportions as specimens of _Isotelus maximus_ from Toronto, the total length would be only 174 mm. The cephalon would be about 52 mm. long, the thorax 58 mm., and the pygidium about 64 mm. long.]

Length of c.o.xopodite of fourth left cephalic appendage, about 18 mm.; diameter, about 2.5 mm. Length of c.o.xopodite of last left cephalic appendage, about 18.5 mm. Distance apart of inner ends of gnathobases of fourth cephalic appendages, about 4 mm. Distance apart of inner ends of endobases of first thoracic segment, about 6 mm. Distance apart of outer ends of c.o.xopodites of first thoracic segment, about 43 mm.

Length of c.o.xopodite of seventh left thoracic appendage 16 mm., diameter about 3.5 mm.; length of basipodite of the endopodite of the same appendage 6 mm.; diameter about 2 mm.; length of ischiopodite 5 mm.; length of meropodite 4.5 mm.; length of carpopodite 4.5 mm.; length of propodite 3 mm.; length of dactylopodite 2.75 mm.; total length of endopodite 25.75 mm.

Length of c.o.xopodite of fourth left thoracic appendage 20 mm., diameter 4 mm.; length of five proximal joints of the endopodite 25 mm.; diameter of basipodite about 2 mm.

RESTORATION OF ISOTELUS.

(Text fig. 9.)

The exopodites have been omitted from this restoration since nothing is known of their actual form. The chief reason for the figure is to contrast the greatly developed c.o.xopodites of the posterior part of the cephalon and thorax with those of other trilobites. The antennules and first two pairs of biramous appendages of the cephalon are more or less hypothetical, and less is known of the appendages of the pygidium than is shown here. The restoration is based somewhat upon Walcott's figure in Science. The outline is that of a specimen of _Isotelus maximus_ from Toronto, Ontario.

=Isotelus gigas= Dekay.

Ill.u.s.trated: Woodward, Quart. Jour. Geol. Soc., London, vol. 26, 1870, text fig. 1; Geol. Mag., dec. 3, vol. 1. 1884, p. 78, text fig. Milne-Edwards, Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, 1881, pl. 12, fig. 46. Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 2, fig. 9; Geol. Mag., dec. 4, vol. 1, 1894, pl.