134 The Ottawa Naturalist. [Nov. 



impression made in the sand, and the ridges correspond to the 

 depressions made by the gnathobases. The trail numbered 3 

 is similar to the one described, but the one numbered 2 is of a 

 different sort. 



The increase in width of the axial lobe, caused by the de- 

 velopment of the gnathobases in the adaptation to the crawling 

 mode of life, explains the parallelism which exists between certain 

 genera of the Asaphidce, and the parallelism between Nileus 

 and some of the Illaanidce. A number of forms, arising from 

 different stocks, were becoming adapted to near-shore life, and 

 all found that enlarged gnathobases assisted them in their new 

 mode of life. Thus the narrow-axised Asaphus produced Oncho- 

 metopus, Isoteloides produced Isotelus, and Symphysurus de- 

 veloped into Nileus, just as some as yet unknown form developed 

 into the Illcenurus of the sands of the Potsdam. 



Locality. This species is represented in the collections 

 at the Museum of the Geological Survey by a thorax and 

 pygidium from Deschenes, and a free cheek from Britannia, which 

 are holotype and paratype respectively. There is also a partial 

 thorax and pygidium collected by James Richardson in 1853 

 from the sandstone of the Chazy at West Hawkesbury. The 

 pygidium of this specimen is 72 mm. long and about 110 mm. 

 wide, and shows a single rib on each pleural lobe. The axial lobe 

 of the thorax is very narrow, being only ii mm. in width. The 

 total width appears to have been somewhat over 100 mm. The 

 fourth specimen was found loose at Point Claire, but was un- 

 doubtedly from the Chazy, which outcrops there. This speci- 

 men shows the posterior portion of the hypostoma and portions 

 of six thoracic segments. The forks of the hypostoma are very 

 short, and the body portion is convex. The axial lobe of the 

 thorax is S3 mm. wide, and the total width of the thorax is 

 160 mm. 



EXPLANATION OF PLATE II. 



1. Bathyurus superbus Raymond. A small cranidium. 

 About natural size. 



2. The same species. An imperfect thorax and pygidium. 

 Slightly smaller ^han natural size. 



3. The same species. A nearly complete specimen. About 

 natural size. 



4. Trail, supposed to be that of an Isotelus. 



5. Impression of the ventral surface of a trilobite, presum- 

 ably Isotelus arenicola. About one-fifth natural size. 



