21 



the cardinal angles compressed and slightly recurved ; umbo very pro- 

 minent, narrowly-rounded, projecting beyond the hinge line ; beak small, 

 Incurved ; area moderately developed, strongly concave, especially under 

 the beak and for a short distance on each side thereof. Deltidium flat, 

 triangular, smooth; its width at the base about equal to the height. 

 When the surface of the area is perfect the dental stride are only indis- 

 tinctly or not at all visible, but where worn or exfoliated they come out 

 to view. They are slightly oblique, the convergence being from the 

 dorsal edge of the area inwards, or inclining towards the beak. The 

 area in the lower, or dorsal, one-third, lies nearly in the plane of the 

 lateral margin, but in the upper two-thirds it is strongly incurved, so as 

 to become more nearly at a right angle to that plane. 



Dorsal valve deeply concave ; nearly in contact with the ventral valve 

 at the cardinal angles, and for a space of about one-fourth the width, all 

 round the sides and front ; the distance between the two valves being the 

 greatest a little above the middle. Area of the dorsal valve nearly as 

 large as that of the ventral, parallel-sided, apparently not flat but 

 slightly convex. It seems to form an obtuse angle with the area of the 

 ventral valve. 



Surface in one of the specimens covered with minute radiating strioe, 

 about twenty in the width of one line. Along with these there are 

 others about twice or thrice their size, and distant from each other from 

 one-fourth to one half a line. No large striae or ribs on the umbo. A 

 second specimen has five or six large ribs, commencing at the beak and 

 dying out before reaching the most prominent point of the umbo. The 

 remainder of the surface covered with minute strice, and larger lines, 

 somewhat like those of the last specimen, but with concave, instead of flat 

 spaces between them. 



Only about a dozen imperfect specimens of this species have been col- 

 lected, and they agree very nearly in form and dimensions with S. demissa, 

 the type of Prof. Hall's genus Strophodonta. The exceedingly prominent 

 umbo, and the minute striasion of the surface, are the only external 

 differences between the two species. One of the specimens being partially 

 silicified, I was enabled to free it from the limestone, with which it was 

 filled by the application of an acid. Beneath the deltidium, under the 

 beak, there is a septum, on each side of which is a deep pit for the recep- 

 tion of one of the branches of the divaricator process. The inner edge of 

 the septum rests upon an arched plate, between which and the shell there 

 is a third deep conical cavity, which extends nearly to the beak. There 

 is no bilobed process. This structure is quite different from that of 

 demissa in which, although there is a rudimentary septum, there is no 



