83 St. Maurice and Ci^aiborne Pelecypoda 83 



V. greggiana Dall, Trans. Wag. Ill, '03, p. 1425. 

 V. ivilcoxensis Dall, ditto, p. 1426, pi. 54, fig. 12. 



If we admit that this stock of Venericardia had in Midway 

 times a similar inherent potentiality for differentiation that the 

 plantcosta stock had, or that as it ascended through the various 

 Eocene stages it was subjected to similar modifications in environ- 

 ment, then, this being a somewhat more highly organized form 

 with greater details of ribbing and less regularity in outline 

 would naturally sooner evolve variations conspicuous enough to 

 be seized upon by authors as constituting ' 'distinct species' ' . 

 Hence the tendency towards listing "varieties" under planicosta 

 and ' 'species' ' under the alHcostata-\\k.e forms. 



The name alticostata was first applied to the good-sized, 

 abundant and rather specialized type of the stock found in the 

 Claiborne ' 'sand' ' at Claiborne. Here it is by far the most con- 

 spicuous member of the genus. Moreover, it is sufficiently vari- 

 able to have caused Lea to separate it into two distinct species, 

 transversa and sillimani, and to cause de Gregorio to institute a 

 new variety, secans. In the red clays in the Orangeburg District 

 of South Carolina, where there is often a commingling of St. Maur- 

 ice and Claiborne species alticostata is present in nearl}^ its usual 

 size, but seemingly with less variation. Our collections from the 

 St. Maurice beds from Alabama westward show no tj^pically de- 

 veloped specimens. 



The general characteristics of alticostata need not be restated 

 here. However, attention may perhaps be directed to a few in- 

 teresting features. 



When young the shell appears somewhat thin and shows 

 within, the position of the external costK. The latter are sim- 

 ple or non-terraced, but are ornamented with little knobs or cren- 

 ules which become long, conspicuous, hollow and irregularly loca- 

 ted on the post-umbonal slope. In the adult the ribs appear as 

 follows : Beginning anteriorly, 1-4, crenulate, simple ; 4 or 5 to 12- 

 15, terraced, crenulate, crenulse becoming funnel-shaped, or as 

 in ftnigina, at the anterio-basal margin ; 12-15 to 17-21, crenula- 

 tion becoming obsolete, terraces becoming sloping and obsolete, 



