151 



measure varying a little) with the posterior edge, which is somewhat 

 straight or gently curved to the most projecting point of this extremity. 

 Ventral margin gently convex or straight for half the length in the middle 

 portion, curving upwards to the posterior extremity, and anteriorly ascend- 

 ing with a broad, rounded curve ; anterior margin descending abruptly to 

 meet the curve of the ventral margin. Sometimes a small anterior ear, 

 but in general the outline from the umbones downward shows no interrup- 

 tion. TJmbones not very large, rounded ; beaks incurved, sometimes in 

 contact, and sometimes 1 or 2 lines apart. From the umbones a broadly 

 rounded gibbosity runs obliquely to the lower posterior angle. 



Surface concentrically marked with strong sub-lamellose lines of growth, 

 often rugose towards the posterior extremity. Shell always very thick 

 and ponderous. 



An average-sized specimen is 2 inches in length and 4 in width. The 

 proportions vary somewhat, some being shorter and more nearly oval than 

 others. 



Locality and Formation. — Cape Smyth, Lake Huron. Hudson Eiver 

 group. 



Collector.— R. Bell. 



Cystodonta Hindi. (N. sp.) 



Fig. 131. 

 Fig. 131. — Cyrtodonia Hindi, a, right valve ; b, dorsal view. 



Description. — Obhquely ovate, or sub-rhomboidal, tapering from the 

 middle to a narrow rounded point at the posterior extremity, strongly ven- 



