TRANSACTIONS OF WAGNER 

 694 



TERTIARY FAUNA OF FLORIDA 



to act as a sucker or scoop-anchor, by which the animal might drag itself 

 about, than as a stilt or vaulting-pole, as in some of the shallow-water species. 



The discrepancy in size of the valves appears to be more or less related 

 to the activity of the animal. The species in which the difference is greatest 

 are probably the more sedentary. In nearly all sessile Pelecypods the lower 

 valve is deeper, and the differences in the Pectens are probably due to the 

 same factors of the environment. 



In nearly all the species the right valve is the least inflated. In a few, 

 which are among the most active swimmers (like /'. irradiaiis Lam.), the right 

 valve is more convex than the left. It does not seem to be a feature of system- 

 atic importance, as species otherwise apparently nearly allied differ in this respect. 



The influence of the environment is very marked among the Pectens. 

 As in mammals and birds, the same species in the northern part of its range is 

 larger than in the south, unless it is a distinctively tropical species. But in 

 color the rule is reversed, the southern specimens being lighter and more 

 brightly tinted than the northern ones in the same species. The specimens 

 which live in deep water and swim actively arc usually thinner-shelled and 

 smoother, while those which inhabit the lagoons are heavier, have more con- 

 spicuous concentric sculpture, and more solid shells. These differences are 

 very marked in our common east coast P. irradiaiis, of which P. dislocates Say 

 is the southern lagoon form ; and parallel differences appear in the similarly 

 related P. vcntricosus and its variety teqnisiilcatiis, on the Pacific coast, and in 

 the fossil P. cboreus and comparilis of the Carolina Tertiaries. 



Whatever might be advisable were our knowledge of the Pectiiiidce con- 

 fined to the recent species, any paleontological division of them cannot ignore 

 the intergradation which is so obvious between the different types, of which 

 the extremes appear so unlike. 



For this reason the subdivisions adopted here will be comparatively few, 

 and their rank such as belongs to groups obviously connected by intimate 

 intergradations of peripheral species. They may be arranged as follows : 

 Subgenus Pecten s. s. Type P. maximns L. 



Left valve moderately inflated, right valve flatfish ; sculpture of strong 

 ribs with radial striation, more or less roughened by simple concentric lamella- 

 tion or incremental sculpture ; ears subequal. 



Section Euvola Ball, 1897. Type P. ziczac L. 



Left valve extremely inflated, surface polished, ribs moderate or obsolete, 



