46 



Transverse length of best specimen 2J inches ; umbones to ventral 

 margin 15 lines. 



This species differs from most others of this genus in tapering to a nar- 

 rowly rounded posterior extremity, and in its undulated surface. 



Locality and Formation. St. Joseph's Island, Lake Huron. Black 

 River limestone. 



Collector. A. Murray. 



CTENODONTA ABRUPTA. (N. sp.) 



Fig. 48. Fig. 49. 



Fig. 48. Ctenodonta abrupta. a, left valve ; b r posterior extremity ; c, hinge line. 

 49. Cyrtodonta Leucothea. 



Description. Sub-triangular, ventricose, umbones large, beaks closely 

 incurved, the posterior extremity abruptly truncated. In the outline, the 

 anterior and posterior dorsal margins form a right angle (very nearly) 

 with each other. The anterior extremity is broadly rounded and is placed 

 altogether in the lower half of the shell. The ventral margin is usually 

 rounded, but is sometimes nearly straight in the posterior half. The poste- 

 rior ventral angle is small, narrowly rounded, and near the ventral margin ; 

 ligament short and very prominent. Surface with concentric lines of 

 growth of variable size, some of them rather fine. 



Width, 6 or 7 lines ; length from umbones to ventral margin, 5 or 6 

 lines ; depth of both valves, 4 lines. 



In the above description I have called the side on which the ligament is 

 placed, the posterior extremity, although it is very abruptly truncated and 

 the beaks seem to curve towards it. 



Locality and Formation. Paquette Rapids, Ottawa River. Black 

 River limestone. Also in the Trenton, at the City of Ottawa. 



Collectors. Sir W. E. Logan, E. Billings. 



CYKTODONTA LEUCOTHEA. (N. sp.) 



Fig. 49. 



Description. Shell small, rhomboidal, greatly alated and abruptly 



truncated posteriorly. Umbones depressed, convex ; beaks closely incurved 



9 but scarcely in contact. Dorsal margin behind the umbones straight, in 



