77 St. Maurice and Claiborne PeIvECypoda 77 



Venericardiaplanicosta Lamarck, PI. 27; PI. 28. Fig.s. 1-3. 



For sj-nonymy and original description, see Bull. Am. Pal., vol. i, ]>. 



172, pi. 14; vol. 2, p. 246, pis. 15, 16. 

 Conrad's Cardita vigintinaria (Jr. Phila. Acad., vol. i, '48, p. 129, pi. 



14, fig. 12) and perhaps C. carolineusis ditto, p. 128, would seem to 



belong there. Both were omitted by Conrad in his list of 1865 



(Amer. Jr. Conch., vol. i.) 



Of the four fairly distinct types oi planicosta recorded from 

 the Sabine (Lignitic) horizons (Bull. Am. Pal. ,vol. 2, pi. 16) 

 two, designated for convenience of reference y and d continue 

 into higher beds, y through the St. Maurice and 6 also through 

 the Claiborne and Jackson stages. Slight modifications of 6 

 reached the old country in mid- Eocene times and were there first 

 described tk.^ planicosta by Lamarck. So far as our collections go 

 it is the English Bracklesham specimens that furnish the nearest 

 replicas of the American forms. At Alum Bay or Whitecliff Bay 

 on either end of the Isle of Wight we have obtained small spec- 

 imens of this .species which are very like those from Texas. 

 Specialized and decadent varieties of this stock tend to broaden 

 and obliterate the costation in later stages of growth. This is es- 

 pecially the ca.se in America. Most specimens of ''densata'" of 

 Conrad (pi. 27, figs. 1-3) show this tendency. It is much better 

 shown in W\^ potapacocnsis of the Maryland Surve)^ Reports. Most 

 persistent of all is this feature in var. K (pi. 28, fig. 3 ; pi. 29, 

 fig. i). However, amongst any and all of these, the reversion to 

 the sharpl}' defined ribbing (pi. 28, fig. i) is very frequent. Con- 

 rad has apparently given two names to these clear-cut forms, 

 vigintinaria and mooreana. 



The American specimens, on the whole, show higher beaks 

 than the European ; especially is this the case with the great 

 early specimens illustrated in vol. 2, pi. 15, or this work. Com- 

 pare fig. I of that plate with fig. 2, pi. 27, of the present volume 

 and it will appear how\ in mid-Eocene time there was a close ap- 

 proaching to the European tj^pe. The beak is not far above the 

 most anterior point in the margin of the shell, and beneath this 

 point there is an oblique truncation of the margin, noticeablj' dif- 

 ferent from the fullness of the margin at this place as shown in 

 form (i, pi. 15, fig. I (vol. 2). 



