316 Silurian Fossils. 



middle, the greatest diameter being at the base of the upper 

 chamber. Near the mouth there is a constriction in the casts 

 caused by the thickening of the shell at this place. The septa are 

 very thin and nearly flat. The siphuncle is very small, and 

 situated close to the dorsal or convex side at the point a. The 

 doreal side is not so much rounded as the ventral, consequently a 

 section across the fossil from a to 5 is elliptical, as seen in Fig. 3. 

 In Fig. 3, the position of the siphuncle is indicated at the point 

 a. The fossil tapers very rapidly to a point from the outer 

 chamber. The ventral side is the straightest. Specimens with the 

 outer shell preserved are covered with fine striae which are slightly 

 flexuous on the dorsal side. 



This interesting little species was first described by Professor 

 Hall, and as it differs from all other known generic forms of the 

 great family of Orthoceratites, he constituted a new genus for its 

 reception with the following characters : Genus Oncoceras, 

 *' tube curved ; aperture constricted ; lower part of the outer 

 <jhamber and upper part of the septate portion, ventricose ; abruptly 

 contracting towards the apex ; si^Dhuncle small, dorsal ; septa plane, 

 nearly flat, slightly elevated on the dorsal margin.*' 



Orthoceras Bilineatum. — Hall. 



Fig. 4. Portion of Orthoceras bilineatum. 



We may add that specimens in the collection of the Geological 

 Survey of Canada shew that the aperture of Oncoceras constric- 

 ium was elliptical, and that the constriction seen in the casts as 

 before stated is caused by the thickening of the shell near the 

 aperture, forming an internal ring at this part of the tube. 



The specific name is Latin, constrictus, narrow or constrained. 

 We have collected specimens of this species at the " Little Chau- 

 diere Rapids, Pauquettes Rapids, and at the City of Ottawa. It 

 appears to be most abundant in the lower part of the Trenton 

 limestone, in the beds reposing directly upon the Black Eiver 

 limestone. 



Fig, 4 is also an Orthoceratite which is never seen of a great 

 •size. The largest specimen we have observed is about eight inches 



