TRANSACTIONS OF WAGNER 

 974 



TERTIARY FAUNA OF FLORIDA 



Genus PSAMMOBIA (Lam.) Bowdich. 

 Psammobia Lam.. An. s. Vert., v., p. ,Sii, 1818; Bowdich, Elem. Conch., ii., p. 6, pi. i, 



fig. 10, 1822. Type P. ferocnsis Gmelin. 



The fossil species in the Paris Basin Eocene appear to show a transition 

 towards Tellina both in general form and in the tendency for the line marking 

 the pallia! sinus to be free from the line due to the attachment of the mantle 

 below it. This is true of the species which in general form are nearest to 

 Psaintnotcea as well as those more like the typical Psammobia. The specializa- 

 tion of form thus increases with the progress of the group in geological time, 

 as ought, on the theory of evolution, to be the case. A careful scrutiny of a 

 large number of recent species shows that the majority of the typical Psam- 

 mobia have the lower line of the sinus nearly or quite coalescent with the 

 main pallia! line ; most of the species of Gobraus show a little more of the 

 sinus free, anteriorly ; some (ex. P. occidcns) have a considerable part of it free ; 

 and in some individuals there is more of it free in one valve than in the other. 

 It appears to be a variable character of very little physiological importance ; 

 nevertheless, the generalization holds good that the Eocene species, as a whole, 

 have the sinus less coalescent than the more recent or the living forms. There 

 does not seem to be any marked difference between the recent and fossil 

 American forms in this respect, but it is obvious in the French fossils. Owing 

 to the variations observed, I am able to regard this character as at most of 

 only subordinate value, though taken into account with other differences it 

 may be recognized as sectional or subgeneric. The hinge-teeth in this group 

 are also rather variable, which is probably due to the fact that these animals 

 are more sedentary than their allies the Tellens, and more given to burrowing. 

 All burrowers, if sedentary, tend to degeneration in such features as hinge- 

 teeth. Throughout the groups the normal formula for the teeth is, ^^^' 

 but this is almost always reduced until the left valve may have but two and 

 the right one tooth. It sometimes happens also that the angle of the cardinal 

 margin in front of the socket for the left anterior tooth may be perceptibly 

 thickened, or even project as a toothlike mass of considerable prominence. In 

 one species I have noticed a projection of the cardinal margin itself before the 

 hinge on one side which is received by a shallow groove on the edge of the 

 valve opposite and behind the hinge, the same in reversed position, which may 

 be regarded as a reminiscence of the lateral teeth of the original Tellina stock. 

 In the very small species the teeth are most reduced, both in size and number. 

 The bifurcation or grooving of the distal end of the chief teeth, though some- 



