36 [Assembly 



second crural processes, a ridge proceeds along the inside of the 

 shell nearly parallel to the margin. This ridge, or thickening of the 

 shell, marks the extent to which the margin of the ventral extends 

 within the edge of the dorsal valve. 



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 lamellae, which, extending into the cavity 

 of the beak, continue downwards, fDrming an elevated rim around 

 the deep muscular impression. This elevated rim, which is conver- 

 gent, 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 mus- 

 cular impression, spreads laterally and becomes slightly raised from 

 the shell, leaving a little cavity beneath it; and in this, and below 

 its edge, are the cordiform adductor imprints*. The median ridge is 

 continued below this point, but less prominently than above. When 

 the muscular impression is perfectly 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 termina- 

 tion of the median septum, and sometimes nearer the beak, and 

 expanding towards the front of the shell. This impression is mar- 

 gined 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 comparison 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 palseozoic Rhynchonellse there are no 

 dental plates visible, and the muscular depression is but faintly defined. / 

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

 ticulates 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 valve and its four crural processes, which differ 

 essentially from Rhynchonella and every other brachiopod known to me. 



* See figures 3 and 4^ page 37. 



