TRANSACTIONS OF WAGNER 



748 



TERTIARY FAUNA OF FLORIDA 



typical irradians, a thinner shell, and more conspicuous concentric lamellae. 

 It is also rather more compressed. Of seventeen specimens two had sixteen, 

 eleven seventeen, and the remainder eighteen ribs. It may be variegated with 

 orange, gray, dark brown, or olive and white, but on the whole constantly 

 averages darker than the southern specimens. It lives in the open bays on 

 weedy or pebbly bottom. A Pleistocene specimen had nineteen ribs. 



Pecten gibbus var. irradians Lamarck. 

 Pecten irrailians Lam., An. s. Vert., vi., p. 173, 1819. 

 Pecten conccntricus Say, Journ. Acad. Nat. Sci. Phila., p. 259, 1822. 

 J'ci It'll tiii-giil us Sowerby, Gen., xxxi., fig. I, 1829 ; not of Lamarck. 



Pleistocene and recent. 



This is the southern and typical form of which borcalis is the northern 

 geographical race. It extends from New Jersey, which is Say's typical 

 locality, south to Georgia and Texas. It lives in open water and usually 

 at a greater depth than the typical dislocatns (= gibbus), and never assumes 

 the bright colors of that shallow-water form, though occasionally variegated 

 in the same manner with dull red. In general its colors are those of the 

 variety borealis. 



Twenty-two specimens had eighteen ribs; twenty-four, nineteen ribs; 

 twelve, twenty ribs ; three, twenty-one ribs ; and four, twenty-two ribs. 

 There is some variation in the roundness or angulation of the sides of the 

 ribs, and there are rarely fine longitudinal striae on the backs of the ribs. 

 The normal number of ribs may be regarded as eighteen to twenty in this 

 variety. The fossils showed nineteen to twenty-two ribs. 



Taking all the varieties together, the generalization may be fairly made 

 that in the Pliocene the proportion of specimens with less than nineteen ribs 

 is decidedly larger than among the recent sFlclls. The variations in the fossils 

 parallel those of the living forms; the concentric sculpture may be weaker or 

 stronger, may be visible on the backs of the ribs or only in the interspaces. 

 The ribs may be more or less emphatic, rounded or flat-topped with lateral 

 angles. In the latter case the concentric sculpture sometimes stops short at 

 the angle, leaving the unworn back of the rib smooth, as if the concentric 

 lamellae had been worn off. In this case, which is the most conspicuous of 

 the various mutations, the ribs appear laterally fringed. All the species of 

 this group, recent or fossil, show this mutation occasionally, though it is rarer 

 among the recent shells than among the fossils. 





