29 



thin mesial septum extends from the beak two-thirds the length of the- 

 shell. For the length of about a line from the beak the septum is much 

 thickened, leaving in the cast of the interior a short, deep fissure, as shown 

 in fig. 1. On each side of this fissure there are two tubercles, more or 

 less elevated, -^vhich are the casts of the cavities in the shell, for the recep- 

 tion of the divaricator processes. The divaricator scars are divided into 

 a number of lobes, usually five or six, but often more. The occlusors are 

 rather small, and extend from the thickened part of the septum about one- 

 third the length of the divaricators. All around the sides and front of the 

 muscular area the casts of the interior are striated, the striae often becom 

 ing coarser as they recede from the margin. 



The casts of the interior of the dorsal valve exhibit two ovate pits close 

 to the beak (fig. Ih,) the impressions of the divaricator processes. From 

 these two pits the impression of a short septum extends, ending in a point at 

 about three lines from the beak. The occlusor scars are situated on each 

 side of this septum, but their form is not exhibited in any of the specimens 

 collected. The shell on each side of the scars is coarsely tuberculated, 

 smooth along the hinge line and at the cardinal angles. The divaricator 

 processes, as shown by their impressions, are short, erect, slightly sloping 

 backwards, but diverging laterally. 



None of the specimens are sufficiently perfect to exhibit all the details 

 of the area. In some the casts show that there is a small triangular pit 

 beneath the beak, as there is in S. magnifica. 



Width on the hinge line from ten to sixteen lines. Length eight to 

 fourteen lines. Specimens two inches wide are sometimes met with. 



This species is, no doubt, closely aUied to S. perplana, Conrad. It dif- 

 fers greatly from that species in the form of the muscular impressions, and 

 it is only on account of this difference that I give it a name; 



Locality and Formation. — Gaspd. In the lower part of the sandstone. 



Collectors. — Sir W. E. Logan, Dr, J. W; Daw^son, R. Bell; 



Strophomena Tullia. (N. sp.) 



Plate 2, figs. 6, Qa. 



Description. — Shell large, nearly flat, semi-elliptical, cardinal angles 

 sometimes slightly extended ; sides in the posterior half straight or gently 

 concave ; anterior half broadly rounded ; width on the hinge line from 

 one-sixth to one-fifth greater than the length. 



Ventral valve very slightly convex, most elevated in the middle or a 

 little above, compressed towards the cardinal angles. Area about two 

 lines high at the beak, inclining backwards at an angle of about 45S to the 

 plane of the lateral margin. 



