13 



open and slightly displaced in the specimen, so that it is impossible to 

 measure the exact convexity through the closed valves. 



This specimen is very similai- in shape to the Anodontopsis angustifrons 

 of McCoy,* from the Upper Ludlow rocks of Westmoreland, but its 

 anterior margin is not so narrowly rounded and its dorsal margin is 

 not arched posteriorly. 



Genus Ilionia, Billings.f 18^5. 

 Canadian Naturalist, 2nd Series, Vol. viii., p. 301. 



" The above generic name is proposed for such forms as Tellinaprisca 

 (Hisinger), Anatina sinuata (Hall), and the species herein described. 

 All the specimens I have seen are internal casts, and the characters of 

 the hinge line, therefore, cannot be given. The form is irregularly 

 ovate, compressed or sub-lenticular; one extremity larger than the 

 other; beaks turned towards the larger end, which is, therefore, 

 supposed to be anterior. In all the species a concave depression 

 commences on the umbones and extends downwards to the posterior 

 ventral margin. A large sub-ovate muscular impression in the upper 

 half of the posterior extremity." Billings. 



Ilionia Canadensis, Billings. 

 Ilionia Canadensis, Billings. 18Y5. Can. Nat., N. Ser., Vol. viii. p. 301. 



Fig. 3. " I^ft side of a cast of the interior of /. Canadensis." 



* British Palseozoic Fossils in the Cambridge Museum. 1855. P. 271, pi. \h, 

 14 and 15. 



t As the original definition of this genus and of the typical species may not be 

 readily accessible to the reader, they are reprinted here, with the figures which 

 accompanied them. 



