TRANSACTIONS OF WAGNER 

 600 



TERTIARY FAUNA OF FLORIDA 



The shell of Pccten comprises two general!}' more or less discrepant valves, 

 united along a long, straight hinge-line by an inconspicuous ligament and a 

 central strong resilium. A single rounded adductor leaves its impression 

 pretty high up, a little before the mesial line of the valves, and the pedal 

 retractors are usually attached to the left valve above it, being often obsolete 

 on the right side. The ends of the resilium are received by subtriangular or 

 oval pits in the umbonal region. These pits may be shallow or deep ; their 

 basal margin sometimes projects slightly into the cavity of the valves; their 

 apex is always nearly coincident with the umbonal point of the valve. In a 

 few species, in the right valve, the lateral margins of the pit are raised into 

 tooth-like processes, which fit into corresponding depressions in the opposite 

 valve (e.g., P. Swiftii Bernh.), but these are not homologous with the so-called 

 teeth of Plicatitla and Spondyhts. Outside of these, radiating fan-like from 

 the apex of the valve, are frequently found one to three pairs of more or less 

 prominent lamina;, which I call the cardinal crura, and further away and 

 below, on the ridges which mark the lower boundary of the ears, will some- 

 times be found another pair, only distally conspicuous, which I have named 

 the auricular crura. The cardinal crura are most conspicuous in heavy shells, 

 especially such as Pecten proper and Lyropccten, and serve to adjust the 

 closing of the valves, as does the hinge armature of the Teleodonts. In a 

 few species the crura are sufficiently prominent to actually interlock with the 

 valves half open ; in many others hardly any trace of them is visible. Almost 

 all species possess in the nepionic stage a well-marked provinculum, formed by 

 an elongated area on each side of the pit, covered by long, narrow, close-set 

 taxodont teeth, separated by narrow grooved interspaces. In most species the 

 provinculum is evanescent or represented in the adult only by faint vertical 

 stria:, which cross the cardinal crura. In a few small, thin-shelled, mostly 

 deep-water species, the provinculum is persistent and functional (c. g., P. thal- 

 assimun Dall), forming an interlocking hinge. In Pccten proper, Clilamys, and 

 some other groups, the upper cardinal margin of the right valve is bent over 

 that of the left valve. There are occasional species in which the adult valves 

 have each a flat area along the whole cardinal margin, covered by the ligament 

 and forming a V-shaped groove between the upper margins of the valves, as 

 in P. Swiftii. The disk of the valve is usually rounded or oblique below and 

 at the sides, but above continued on each side in a straight line to the umbo. 

 The shell adjacent to these straight lines is frequently slightly different in 

 sculpture from the rest of the disk, forming narrow areas, which were called 



