HUDSON-RIVER GROUP. 259 



ADDENDUM TO THE TRILOBITES. 



Among several western species not included in the previous descriptions, I introduce the 

 following, for the purpose of illustrating the characters of a genus proposed by Mr. Conrad, 

 and of showing a new form associated with several well known Trenton species, in the 

 lower limestones of the West. , 



Genus THALEOPS (Conrad). 



"Ovate, profoundly trilobed, lateral lobes wider than the middle lobe ; buckler lunate, 

 with very remote oculine tubercles, not reticulated ; abdomen with ten articulations ; ribs 

 without grooves, and not alternated in size ; outer half of lateral lobes suddenly depressed ; 

 post-abdomen without ribs or grooves, and profoundly trilobed." 



I am not able to perceive the essential difference between this fossil and Ill^nus, of 

 which it possesses all the characteristics, in the number and character of the articulations 

 of the thorax, in the cephalic and caudal shields, and in the direction of the facial suture. 



313. 1. THALEOPS (ILL^NUS) OVATUS. 



Pl. LXVII. Figs. 6 a. b. 

 Thaleopt otata. Conrad, Proc. Acad. Nat. Sci. Philadelphia, Vol. i, 1843, p. 332. 



" Obtusely ovaie ; surface minutely punctate ; head very wide, lunate, involuted ; eyes 

 very prominent, rounded, smooth, placed on a line with the angle in the middle of the 

 side lobes; ribs flattened, smooth, without a border at the extremities, where they are rounded 

 and not expanded ; post-abdomen with the middle lobe convex, rounded and well defined 

 at the extremity; inferior margin obtusely rounded. Length two-thirds of an inch; width 

 of the buckler three-quarters of an inch." 



This species is clearly distinct from either of those previously described, in the pro- 

 portional width of the lobes of the thorax ; and it is remarkable for the trilobate character 

 of the caudal and cephalic shields. The outer surface of the crust is punctate, and not 

 striated, as in several other species. The eye is remarkably prominent, and somewhat 

 truncato-conical, apparently smooth to the naked eye, and presenting a finely punctate 

 surface, only, under the magnifier. 



This species is associated with another species of Illanus ( I. crassicauda ? ), Ceraurus 

 pleurexanthemus, Phacops callicephalus, or an allied species, as well as other well known 

 Trenton limestone fossils. 



Fig. 5 a. The specimen of the natural size. 



Fig. 5 b, A part of the cephalic shield magnified, showing the punctures and the course of the facial 

 suture. 



33* 



