95 vSt. Maurice and Claiborne Pklecypoda 25 



"Shell small, thin, with eight large ribs, which near the umbo are di- 

 vided by one or two well-marked sulci, which soon become obsolete, after 

 which the ribs are simple ; the surface sculpture is of even, uniform, crowd- 

 ed, concentric elevated lines. The ears are subequal, the byssal notch well 

 marked". 



The sulci mentioned by Dall are caused by a tendency on 

 one or both sides of the big ribs to form .secondarj' riblets. This 

 is especially noticeable near the umbo, but reappears near the 

 margin as implied b}' Aldrich and Meyer's description. 



We do not find the number eight so rigidl}' fixed for the ribs 

 as former descriptions imply. Pcden clavatiis of the Mediterran- 

 ean is closely allied to this form. Aldrich and Me3'er seem to 

 have had only the right valve ; Dall, only the left. By a slight 

 lapsusiw the latter's work it is referred to the Jacksonian Eocene. 



Type. — Aldrich Coll., Johns Hopkins Univ. Mus. 



Horizon. — St. Maurice Eocene. 



Specimens figured. — Cornell Univ. Mus. 



Locality. — Wautubbee. 

 Pecten clarkeanus Aldrich, PI. 15. Figs. 8-13. 



P. clarkeanus Aldrich, Bull. Am. Pal., vol. i, p. 68, pi. 6, fig. u, 1895. 

 P. frontalis VidiW, partiui. Trans. Wag., Ill, p. 753, '98 



For Aldrich's original description, see Bull. Am. Pal., vol. i, p. 68, as 

 cited above. 



This is a remarkably interesting and variable species as Aid- 

 rich has already pointed out. The vast number of 3'oung or 

 small forms show the beautifully fine and distinct radiation about 

 the umbo though with but .slight traces of radiate sculpturing 

 over the general surf ace of the shell (figs. 10, 12). Camptonectes 

 marking is, however, well defined. Occasionally the radiate 

 sculpturing expands and lengthens and covers the entire surface 

 of the shell (figs. 8, 13). Again it appears about the margins 

 ov\y (fig. 9.) The shell substance is thin, and as Aldrich re- 

 marks, recalls P. scintillatus. At the tj'pe locality only, Sowilpa 

 Cr., have we observed (in specimens kindly loaned b}' Aldrich) 

 the particular form of ribbing .shown on pi. 6, vol. i of these 

 Bulletins. (See also pi. 15, fig. 8, of the present work). Even 

 these specimens are not so thick and solid looking as the figure 



