Raymond-Narrawav : Notes on Ordovician Trilobites. 57 



Axial lobe of thorax more than oiic-lhird and less than one-half the 

 total width, only slightly convex. 



Pygidiuni evenly convex, rounded in outline, seven-tenths as long as 

 wide. Th margin has a wide concave border. The axial lobe is distinct 

 but not i)ronunent. 



Length of cranidium, 30 nun.; width of thorax at back of fourth segment 

 44 nun.; width of axial lobe at same point, 19 mm.; length of pygidium, 

 32 mm. ; width 45 mm. This s|:)ecimen is from the Lowville at Mechanics- 

 viUe. 



The second specimen is slightly smaller and retains the posterior part 

 of the cephalon with one eye, all the thorax and the pygidium. There 

 are no spines at the genal angles, the eye is small and high, and the axial 

 lobe of the pygidium is distinct and shows seven pairs of shallow pits on 

 the anterior half. 



The axial lobe of the thorax at the back of the fourth segment is 15 mm. 

 wide, the total width being 35 mm.; the pygidium is 25 mm. long and 34 

 mm. wide. This specimen is from the Black River at Mechanicsville. 

 Both specimens are in Mr. Narraway's collection. 



These two specimens retain characters lost early in life by Isotelus gigas, 

 namely, the narrow axial lobe of the thorax and the rounded pygidium. 

 It is significant that they were found in the Lowville and Black River. 

 A pygidium from the Lowville at Newport, New York, seems to belong 

 to this species. Its dimensions are: length 31 mm., width 44 mm.; the 

 length being .70 the width, as in the specimens from Mechanicsville. The 

 pygidia of Isotelus gigas having this same ratio are from 10 to 20 mm. long. 



ExPL.\N.\TioN OF Plates. 

 Plate XV. 



1. Isotelus gigas Dekay. A specimen from the Black River at Pattersonville, 

 New York. This specimen shows the hypostoma in position, the subtriangular 

 outline of cephalon and pygidium, and the small genal spine of an almost mature 

 individual. About natural size. Specimen in the Carnegie Museum. 



2. The same species. A small specimen from the Black River at Ottawa, 

 Canada, in Mr. Narraway's collection. Three-fourths natural size. 



3. Isotelus gigas? A specimen with a narrower axial lobe and a wider, shorter, 

 and more rounded pygidium than is usually seen in Isotelus gigas. The specimen 

 is from the Lowville limestone at Ottawa, and is in Mr. Narraway's collection. 

 About natural size. 



