186 



At first sight the more perfect of these two specimens has the general 

 appearance of a Ctetiodonta, and its marginal outline is somewhat like 

 that of C. dubia (Hall) as figured by E. Billings on page 175 of the 

 " Geology of Canada." On closer examination, however, its hinge would 

 seem to have been practically edentulous, and therefore very difierent to 

 that of Ctenodonta. The shell, also, has much the aspect of a small, nar- 

 row species of Thracia, but in that genus the left valve is always fiatter 

 than the right, and the ligament internal. The species is here provision- 

 ally referred to Clinopistha, at the suggestion of Mr. Ulrich, but it may 

 represent a new generic type, which at present there are not sufiicient data 

 to define. In the preceding description of the characters of the species, 

 the shorter, narrower and truncated or subtruncated portion of the shell 

 is regarded as the posterior side, and vice versa, in accordance with what 

 are presumed to be its natural homologies. 



Ehytimya recta. (N. Sp.) 

 Plate 20, fig. 7. 



Shell elongated, more than twice as long as high, and very inequilateral: 

 valves compressed laterally, most convex but not very prominent along 

 the very oblique and indistinctly defined posterior umbonal slope and 

 shallowly inflected above it. Anterior side extremely short, its margin 

 sloping obliquely and rapidly downward and outward in the lunular 

 region, most prominent and forming a narrowly rounded or somewhat 

 pointed junction with theventT-al margin, at a little below the midheight : 

 posterior side about five times as long as the anterior : posterior end 

 obliquely truncated, and forming an obtusely pointed junction with the 

 ventral margin, which is nearly straight or but very slightly convex for 

 the greater portion of its length : cardinal margin, behind the beak, 

 straight, horizontal and almost parallel with the ventral border : umbones 

 compressed and slightly depressed both laterally and vertically, beaks 

 incurved, placed very nnar to the anterior end, but not quite terminal. 



Surface of the cast marked with irregularly disposed but continuous 

 concentric strias or lines of growth, and by a few short faint undulations, 

 parallel to them, on the posterior umbonal slope. 



Hinge dentition and muscular impressions unknown. 



Dimensions of the only specimen known to the writer: maximum length, 

 38 mm.; height, at the midlength, 15'2 mm. 



A perfect and tolerably well preserved cast of the interior of the right 

 valve of a specimen of this species was collected by Mr. Dowling in 1890 

 from a loose piece of limestone at Reindeer Island, Lake Winnipeg. 



