18 University of Texas Bulletin 



become nearly parallel to the ridge. The opposite side of 

 the beak always appears to be entirely smooth, which prob- 

 ably is in part due to the state of preservation. The ribs 

 are crossed by lines of growth and by strong lamellae which 

 give them a scaly aspect. The lines of growth and lamellae 

 'cover also the beak. 



The ligamental groove is generally spirally curved, deep, 

 broad, triangular, very long to very short, according to the 

 shape of the beak, limited on both sides by a ridge. Below 

 the groove on the posterior side is a shelf -like fold finely 

 crenulated, on the anterior side a similar one but rounded 

 and less distinct. The shell outside of these ridges is also 

 finely crenulated along the upper part of the animal cavity, 

 the crenulation becoming very faint on the lower half until 

 it disappears, entirely. The muscular impression is deep 

 and very near the posterior margin. The cavity often ex- 

 tends somewhat into the beak. 



Upper valve. 



Rather thick, slightly convex to flat, operculiform, sub- 

 ovate with a nearly flat spiral twist, the beak being near 

 the margin. Beak very little prominent, nearly pressed 

 down to the rest of the shell. Sometimes there is a very 

 obtuse, spirally curved umbonal ridge which disappears to- 

 ward the postero-inferior margin. On the ' posterior side 

 right near the beak is a distinct depression. 



On a well preserved specimen the ornamentation consists 

 of numerous concentric growth lines and lamellae which 

 cover the whole surface. The lamellae are not of circular 

 form, but show an angular prolongation on the umbonal 

 ridge. Where this is absent the lamellae follow a more or 

 less elliptical course. Fine radiating ribs are visible on the 

 postero-superior part of the shell in the region close to the 

 beak. The margins are distinctly crenulated. 



The ligamental groove is mostly broad and flat, seldom 

 narrow and deep ; in the first case strongly spirally curved, 

 in the second case much less so. In the first case, the lig- 

 amental groove is almost parallel to the superior margin; 

 in the second case it nearly forms a right angle with it. 



