Rocks and Fossils of Phillipshurgh, C, E. 319 



more than the width, the difference being caused by the variable 

 form of the front margin which is sometimes concave or nearly 

 straight, as represented by the figures a and b, (below) while often 

 the middle portion is either convex or projects so as to form a 

 small rounded lobe. The ventral valve is either moderately or 

 strongly convex ; the beak pointed and slightly elevated above the 

 hinge line, with a small area beneath it ; the hinge line somewhat 

 straight, its length about half the width of the shell ; sides round- 

 ed ; the front margin either concave, straight, or convex, some- 

 times with a small projecting lobe in the middle ; the mesial 

 sinus is usually one-third the width of the shell, evenly rounded 

 in the bottom, and becoming obsolete before reaching the beak. 

 The dorsal valve is more uniformly convex than the ventral ; the 

 mesial fold rounded and usually disappearing at about half the 

 length of the shell. The surface has usually a smooth appearance, 

 but on many specimens from ten to twenty concentric sublamellose 

 ridges of growth are visible. 



Fig. 3. 



Fig. 3. — Camerella calcifera ; a, ventral valve; h, dorsal; c, interior 

 of ventral valve, shewing the small chamber beneath the beak. 



Affinities of this species. — Camerella extans (Emmons) has 

 the hinge line wider and the mesial lobe defined to the beak. 

 C. nucleata [Atrypa nucleus') is most closely allied to this species, 

 but is in general more strongly trilobed, and, according to Prof. 

 Hall, has the beak of the ventral valve incurved over that of the 

 dorsal. In our species it is elevated in a manner similar to that 

 of the beak of an Orthis. Notwithstanding these differences 

 these three species are all closely related and may yet be united. 



It is to be borne in mind that this species varies greatly in 

 size and in the contour of the front margin. The mesial fold on 

 the dorsal and the sinus in the ventral valve are sometimes nearly 

 obsolete, but in general are well developed for half the length of 

 the shell. Out of about 100 specimens which I have examined 

 there are three in which the sinus extends nearly to the beak, but 

 in all the others it dies out about the middle of the shell. 



Although the individuals of this species are numerous, I have 

 not succeeded in getting a specimen with the valves united. 



