- 2 '-4 - 



ant. In shape and thickness the usual variation thrt character- 

 izes attached hivalves is encoimterefl ; hut, on the whole, the 

 valves are of raediiim thickness ; or less roimdecl in on.t- 



line. •'•ho absence of plications on either a portion of the sur- 

 face of the entire surface of the shell depends upon the size of 

 tie area of attachment and since the right or left valve may be 

 attached, eithei' valve ma;- ''^" tartly or entirely smooth. '■'•'hG 

 average number of plications • is siTc or seven. Hone of the speci- 

 mens have short, rnar?:inal plications intercalated between the 

 prir.ery ones. This feature, together with the small size, anr!^ 

 thinness of the shell, sug??ests that the Bovvden forms may be a 

 disti'iCt variety. Their salient features are the small size, rather 

 broad, subrounded outline and the relatively numerous, somewhat 

 foliaceous plications. The Iliocene (Virginia, ITorth Oarolina) and 

 Pliocene (North and South Carolina^ Florida) P. mar^-inata Say (l). 



(1). Jour. Acad. Hat, :ici., Philadel-ohia, vol. 4, n-o. 156-1^^7^ 

 pi. 9, fig. 4, 1824. 



is more triangular in outline, mth fei^er but more prominent pli- 

 cations, the same :^iff erer.ces distinguish -"^ensata from the recent 

 Antillean ?. gibbosa Lamarck (2), and the recent form, is, in addi- 



{?]. Syst. iiniri. sans Vert., 'o. 132, 1801. 



