4«4 PALaJONTOLOGY OF NEW-YORK. 



In the ventral valve, the two strong teeth proceed from the thickened 

 margin of the valve ; and below these, but not distinctly connected with 

 them, are lamellaa, which, extending into the cavity of the beak, continue 

 downwards, forming an elevated rim around the deep muscular impression. 

 This elevated rim, which is convergent, grasps the neck of the cardinal 

 process of the opposite valve in its narrowest part between the two pairs 

 of processes (figures 1, 2, 3, and 6). 



The muscular area is longitudinally oval, with a strong median plate, 

 which, about halfway from the beak to the base of the muscular impres- 

 sion, spreads laterally and becomes slightly raised from the shell, leaving 

 a little cavity beneath it ; and in this, and below its edge, are the cordi- 

 form adductor imprints*. The median ridge is continued below this point, 

 but less prominently than above. When the muscular impression is per- 

 fectly preserved, it is radiatingly plicated towards the margin in all the 

 species observed. The casts of the ventral valve show the form of this 

 muscular impression in strong relief, and, in well-preserved specimens, 

 the small adductor impression projects a little above the level of the other 

 part. 



The muscular impression in the dorsal valve is somewhat central, oval 

 or cordiform ; beginning sometimes a little above the termination of the 

 median septum, and sometimes nearer the beak, and expanding towards 

 the front of the shell. This impression is margined by a slightly elevated 

 rim, and in some species there is a double rim. 



The points of similarity with Rhynchonella will at once be observed on com- 

 parison of this description with that of the former genus, or on comparison with the 

 fossils themselves. The most striking difference in the ventral valve is the absence 

 of dental plates, strictly speaking; though these are represented in the elevated 

 lamellae surrounding the muscular impression, which is much stronger and differs 

 in some respects from that of Rhynchonella. In some of the palaeozoic Rhynchonellse 

 there are no dental plates visible, and the muscular impression is but faintly defined. 

 In the median septum of the ventral valve, which in the older shells articulates 

 with the central process of the opposite one, there is a character not observed in 

 Rhynchonella proper. The most conspicuous difference, however, is in the dorsal 



Sec flgurei 8 and 4, pa|j;e 48t>. 



