79 St. Mauricu and Claibornk Pei,ecypoda 79 



pi. 14, fig. II. Conrad omits this name in his list of '65. 



Lea'' s original description. — Shell rotund, inequilateral, slightly in- 

 flated, longitudinally and closely ribbed ; substance of the shell rather 

 thick ; lunule small, ovately cordate ; beaks slightly elevated, recurved ; 

 ribs about twenty-six and furnished with closely set arched scales ; teeth 

 rather oblique ; cicatrices rather impressed ; cavity of the shell somewhat 

 deep and rounded ; margin rather largely crenulate. 



Diam. .6, Length i. Breadth i inch. 



Observation. — This fine species is remarkable for its beautifully fur- 

 nished ribs, which are so thickly set with arched scales, as almost to cause 

 them to lie over each other. 



De Gregorio believes this to be a variet}' of alticostata. Dall 

 and Cossmann, however, regard the two distinct. So far as the 

 Claiborne specimens are concerned, there is little reason for unit- 

 ing the two, though, that the}" belong to the same stock of the 

 genus cannot be denied, and when traced back into the St. Maur- 

 ice stages great difficulty often arises in endeavoring to separate 

 the ancestral forms of the two. 



To begin with, there are two varieties of rotunda in the Clai- 

 borne ' 'sand' ' . One with its ribs bedecked with ' 'arched scales' ' , 

 the one Lea had in mind in describing the species, reminding one 

 of a series of minute, inverted funnels, or perhaps a dead branch 

 bedecked with a series of saucer-shape fungi, hence we suggest 

 for it the varietal name />c;/^/wfl! (pi. 28, figs. 4,5). The second, 

 slightly more abundant and growing at times a little larger than 

 the first, is practically the t5-pe Lea figures in his Contributions to 

 Geology. The ribs are ornamented by simple nodules, giving a 

 more cord-like aspect, hence the varietal uSinx^ funiculus (pi. 28, 

 figs. 6, 7). 



In such a regular form as this, one can scarcely speak of an 

 innbonal ridge, but, in the general region from beak to postero- 

 basal margin there is a tendency to somewhat suppress costal 

 ornamentation . Two or three ribs beneath the ligament margin 

 show rather exaggerated ornamentation (pi. 29, fig. 5). In the 

 medial portion of the shell the ribs show more or less clearly that 

 the}^ rest on a broader, basal ribbing, i. e. shows terrace struc- 

 ture. 



