34 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Finally (1918, p. 133, pi. 24, figs. 3, 3a; pi. 25), Walcott has redescribed the speci- 

 men from Ohio, presenting a new and partially restored figure. He refers also to the speci- 

 men from Ottawa under the name Isotelus covingtoncnsis? Foerste (not Ulrich). He 

 advances the view, which I am unable to share, that the cylindrical appearance of the 

 segments of the appendages of Isotclus is due to post-mortem changes. 



Isotelus latus Raymond. 

 (PI. 10, fig. i.) 



Illustrated: Asaphus platycephalus Billings, Quart. Jour. Geol. Soc., London, vol. 26, 1870, pi. 31, figs. 1-3; 

 pi. 32, figs, i, 2. Woodward, Geol. Mag., vol. 8, 1871, pi. 8, figs. I, la. Gerstacker, in Bronn's "Klassen u. 

 Ordnungen d. Thier-Reichs," 1879, pi. 49, fig. i. von Koenen, N. Jahrb. f. Min., etc., vol. i,' 1880, pi. 8, 

 fig. 8. Milne-Edwards, Ann. Sci.- Nat., Zoologie, ser. 6, vol. 12, 1881, pi. 12, fig. 45. 



Isotelus latus Raymond, Bull. Victoria Mem. Mus., Geol. Survey Canada, No. i, 1913, p. 45 (species 

 named). 



Isotelus covingionensis? 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 speci- 

 men, 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 coxopodite and a trace 

 of the endopodite. The outer end of the coxopodite is close to the outer margin of one 

 of the prongs of the hypostoma and about 3 mm. in front of its posterior end. The gnatho- 

 base curves backward and inward, and appears to pass 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 frag- 

 ment about 2 mm. back of the center is really a part of a gnathobase. 



The specimen retains only the coxopodite and basipodite of the posterior cephalic ap- 

 pendage on the left side. The coxopodite 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 coxopodites only, but most with more or less poorly preserved endop- 

 odites as well. No exopodites are visible. The coxopodites 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 ma.vimns, and the 

 segments, while of less diameter than the coxopodites, do not show so great a contrast to 



