TRANSACTIONS OF WAGNER 

 696 



TERTIARY FAUNA OF FLORIDA 



grooves corresponding to the external ribs ; Camptonectcs striation present, 

 but usually obscured by the radial sculpture; ctenolium and byssal notch 

 obvious. 



Section Plagioctcnium Dall, 1898. Type P. ventricosits Sby. 



Resembling sEqitipectcn but without radial striation ; the concentric 

 sculpture in looped lamellae; the ribs strong, frequently smooth above; the 

 submargins impressed below the subequal auricles; the valves well inflated 

 with a tendency to oblique growth in the adult. 



To this very natural group belong nearly all the shallow-water Pcctens 

 of our own coasts, such as P. irradians Lam., P. gibbits L., P. dislocatns Say, 

 P. ventricosus Sby., P. nucleus L., P. pnrpitralns Lam., P. cborcus Conrad, P. 

 comparilis T. and H., and numerous other fossil species. 



Section Pallium Schum., 1817. Type P. plica Lam. 



Shell with the disk high and narrow above, ears small ; valves moderately 

 inflated, nearly similar, the basal margin in the adult contracted, so that the 

 edges meet each other nearly vertically ; ribs few, large, widening distally, 

 entire; surface radiately imbricately striate, frequently with Camptonectes 

 striation and imbricate external layer; the cardinal crura usually well 

 developed, often irregular. 



The developed cardinal crura are a function of the short hinge-line and 

 of little systematic importance ; their regularity is usually exaggerated in the 

 figures. Pectcn pallium L. is more appropriately placed in the Chlauiys section. 

 In the typical species of this group the most peculiar features arc the perfectly 

 closed valves, the margins meeting all around the shell, the absence of a 

 byssal notch or fascicle, and the obvious tendency of the irregular cardinal 

 crura to be discrepant on the two sides of the rcsilial pit. Some specimens 

 have very regular lamina? like those of most laminate species, in others the 

 anterior laminae show a tendency to break up into nearly vertical narrow folds. 

 These features are apparently confined to the less normal specimens of this 

 species and should hardly form the basis for systematic rank. Pectcn pana- 

 mcnsis Dall, which has in most respects an unusually close resemblance to /'. 

 plica, differs by having the cardinal laminae obsolete and in the presence of a 

 byssal sinus and ctenolium. In fact, the interchange of characters is so multi- 

 farious that one must, to be consistent, either propose a genus for every two 

 or three species of Pectcn or include all the species in one generic group. 

 Locard, who adopts the former method, has proposed the name Felipcs for a 



