64 Annals of the Carnegie Museum. 



the term a useful one. The axial lobe is somewhat wider than in Asaphus 

 and the glabella is much smoother than in that genus. The form of the 

 trilobites to which this term is applied is much like that of Isotelus. In 

 fact, the parallelism is so close that the differences may not be evident 

 to many persons until they have examined well preserved specimens. 

 The close relationship to Asaphus is at once revealed by the absence of a 

 concave border on cephalon and pygidium. Beside the Onchometopus 

 of the Chazy, the subgenus is represented in America by 0. simplex 

 Raymond and Narraway of the Black River, and Asaphus siiscz Whitfield 

 and Asaphus alcer Billings of the Cincinnatian. 



Onchometopus obtusus (Hall). 



Plate XVIII, figures 2, 3, 4. 



Asaphus ? obtusus Hall, Paleontology New York, I, 1847, 24, pi. 46/5, fig. 14. 

 Isotelus obtusus Raymond, Annals Carnegie Museum, III, 1905, 344, pi. 12, figs. 

 I, 2. 



This species, which can now be more fully described from a nearly 

 complete specimen sent by Professor Perkins, should be removed to the 

 genus Onchometopus, the most striking generic character being the absence 

 of a channeled border on the cephalon and pygidium. 



Description. 



Entire animal oval in outline, rather broad and only slightly convex. 

 The entire test is very coarsely punctate, making the identification of the 

 species easy. 



Cephalon nearly semicircular in outline, being about one-half as long 

 as wide. It is gently and regularly convex, and lacks the depressed or 

 concave border seen in the associated Isotelus harrisi and Isoteloides 

 angusticaudus. The glabella is faintly defined, nearly smooth, three 

 pairs of very shallow furrows being visible on the cast. The neck-furrow 

 is shallow, and hardly visible on the free cheeks. Just in front of it is a 

 small median tubercle. The eyes are small and are situated about their 

 own length in front of the posterior margin. Free cheeks with rounded 

 genal angles. 



The thorax has eight rather broad flat segments which are abruptly 

 deflected at the sides. The median lobe is about one-third the total 

 width of the thorax, being wider than in Asaphus and less wide than in 

 Isotelus. In figure I, of [Plate XII, of my previous article the axial lobe 



