ISOTELUS. 37 



One point of considerable interest in this specimen is the thickness, as it probably gives 

 some measure of the space occupied by the animal. In Triarthncs and other trilobites from 

 Rome, New York, the appendages are pressed directly against the dorsal test, but in 

 this specimen a considerable space intervenes between the plane of the appendages and the 

 shell. 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 esti- 

 mated 'from the mould of the ventral surface, no mm.; Walcott's restoration, 105 mm. 



Length of coxopodite of fourth left cephalic appendage, about 18 mm.; diameter, 

 about 2.5 mm. Length of coxopodite of last left cephalic appendage, about 18.5 mm. Dis- 

 tance apart of inner ends of gnathobases of fourth cephalic appendages, about 4 mm. Dis- 

 tance apart of inner ends of endobases of first thoracic segment, about 6 mm. Distance 

 apart of outer ends of coxopodites of first thoracic segment, about 43 mm. 



Length of coxopodite 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 car- 

 popodite 4.5 mm.; length of propodite 3 mm.; length of dactylopodite 2.75 mm.; total 

 length of endopodite 25.75 mm - 



Length of coxopodite 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 coxopo- 

 dites 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 hy- 

 pothetical, 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. 



Illustrated : Woodward, Quart. Jour. Geol. Soc., London, vol. 26, 1870, text fig. I ; Geol. Mag., dec. 3, 

 vol. i, 1884, p. 78, text fig. Milne-Edwards, Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, 1881, pi. 12, fig. 46. 

 Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pi. 2, fig. 9; Geol. Mag., dec. 4, vol. I, 1894, pi. 

 8, fig. 9; Proc. Biol. Soc. Washington, vol. 9, 1894, pi. I, fig. 9. 



The specimen in the British Museum which Woodward called Asaphus platycephalus, 

 is, in all probability, an Isotelus gigas. Woodward says of it: 



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. 



