Raymond-Narraway : Notes on Ordovician Trilop.ites. 53 



A single craiudiuin belonging to this sjiecies has been found in the 

 Black River at Ottawa. It is fairly common in the Black Ri\'er at 

 Pattcrsonville, New York, and in the lower part of the Trenton (zone 

 with J\i>tislroplii(i hcviiplicdla), at Smith's Basin. Both cephala and 

 p>gidia are a|)t to be confused with Isolelus gigas, but the pygidia are 

 more convex, more triangular, and have a narrow, prominent axial lobe. 

 The glabella is obscurely outlined, there is a small median tubercle just 

 in front of the neck-ring, and a pair of shallow, oblique glabellar furrows 

 between the eyes. On the casts of the interior three pairs of glabellar 

 furrows are faintly indicated. Tiie axial lobe of the thorax is not so wide 

 as in Isolelus, being about one-third the total width. 



This s])ecies is referred to Isoteloides instead of Isolelus because of its 

 fainiK- outlined glabella, the presence of glabellar furrows and a median 

 pustule, and l)ecause of the narrow axial lobe. The hypostoma has not 

 yet been seen. 



This species is more closely allied to Isoteloides angusticaudus Raymond 

 of the Chazy than to the type of the genus. 



Genus IsoTELUS Dekay. 



Isotelus gigas Dekay. 



Plate XV, figures i, 2. 



Isotelus gigas Dekay, Annals Lyceum Natural History New York, I, 1824, 176, 

 pi. 12, fig. I. — H.'^LL, Paleontology New York, I, 1847, 231, pis. 60-63. 



Xo attempt is made to give the full synonymy of this species. 



Ontogeny. 



The collections made by Mr. Narraway from the Black River near 

 Ottawa contain a number of small and nearly complete specimens of this 

 species which make it possible to obser\-e with some completeness the 

 later stages of growth. 



The smallest specimen consists of a pygidium with four thoracic seg- 

 ments attached. The pygidium is three millimeters long and five wide, 

 and the axial lobe of the thorax occupies exactly one-third the total width. 

 The pygidium is nearly semicircular in outline, with a concave border. 

 The axial lobe is very prominent and extends to this concave border. 

 There are traces of six rings on the axial lobe, and three pairs of faintly 

 outlined ribs on the pleura. Some pygidia a millimeter longer than this 

 one show the segmentation more strongly than this particular specimen. 



