114 The Ottawa Naturalist. [Sept. 



of these specimens, in which the shorter, higher and deeper 

 portion of each valve is regarded as anterior, and the prolonged 

 portion as posterior. 



Shell compressed convex, about one-third longer than 

 high, narrowly subovate and very inequilateral. 



Anterior (?) portion of each valve short, its margin shallow ly 

 concave above the midheight, convexly curved and narrowing 

 rapidly inward to the base below ; posterior ( ?) portion much longer 

 than the anterior, decreasing gradually both in height and 

 depth, its extremity obliquely subtruncate above and narrowly 

 rounded below. Ventral margin convexly curved anteriorly, 

 almost straight but slightly concave behind. Cardinal border 

 descending very gradually behind the beaks and much more 

 rapidly so in front of them ; umbones broad and more or less flat- 

 tened; beaks appressed, incurved, with a forward (?) inclination, 

 placed considerably in advance of the midlength. 



Surface marked with numerous, fine, close-set, concentric 

 raised lines, or minute narrow ridges. Impressions of the hinge 

 teeth minute and very indistinctly defined in all the specimens 

 that the writer has seen. On the shorter and presumably an- 

 terior portion of the hinge line there appear to be about four 

 of these impressions ; and on the longer and presumably pos- 

 terior portion, about twice as many. 



Associated with these specimens there are quite a number 

 of badly preserved casts of the interior of the closed valves of 

 a shell which may be referable to this species, but which show 

 no impressions of any of the hinge teeth. These casts have much 

 the same marginal outline as the typical C. parvidens, but in 

 some of the former the valves seem to be proportionately more 

 convex, the umbones farther apart, and the beaks more curved. 



EXPLANATION OF PLATE IIL 



(The originals of all the figures on this plate are mere casts of the 

 interior of the shell, and the whole of the figures, except figs. 7 and 14, 

 are of the natural size. The originals of figs. 4-15, both inclusive, are 

 from the Chazy sandstone at the Hog's Back). 



MoDioLOPsis PARViuscuLA (page 106). 

 Fig. 1. A right valve of this species, from Cornwall, Ontario. 

 Fig. 2. A left valve, from Punk Island, Lake Winnipeg. 



Cyrtodonta breviuscula (page 107). 



Fig. 3. The type of this species, a left valve, from three miles east of 

 Ottawa. 



