TRANSACTIONS OF WAGNER 

 602 



TERTIARY FAUNA OF FLORIDA 



deposited on the angle internally, forming lira;, coincident with the angle of 

 the shell between the ribs and interspaces. This liration becomes habitual 

 in some species. If then in the evolutionary progress of these forms it 

 happens that the external ribbing becomes obsolete, the linu may be retained 

 by natural selection, as useful in strengthening the flattened disk, and thus we 

 have the internal line of Amu sin in accounted for. It is not at all uncommon 

 for ribbed species to have a smooth or obsoletely ribbed variety, and among 

 the Eocene species here described is one which, within the species, shows 

 every stage of the transition between a ribbed Pcctcn and an internally lirate 

 Ainnshtm, thus rendering it impracticable to assign the latter group a system- 

 atic value greater than that of a subgcnus. The same thing may be observed 

 in a good series of the recent P. kyalinus Poli. While the lira; may appear 

 without relation to any external sculpture in their final stage, there seems to 

 be no doubt that, at the time of their inception, they were absolutely depen- 

 dent upon a particular kind of external ribbing or fluting. 



In the obsolescence of ribbing the right anterior ear, probably because 

 of strains resulting from the adjacent byssus making special strength necessary, 

 usually is the last to lose its radial ribbing, and often retains it after the rest of 

 the shell is practically smooth. 



Apart from the ribs or riblets, which usually cover the surface of the 

 disk and ears, there are two other forms of external sculpture to be noted. 

 One of these, originally supposed to be exclusively characteristic of the 

 ^r^ genus Camptoncctcs, is composed of fine, almost microscopic, more or less 

 vermicular groovings, which radiate from the umbo and are deflected laterally 

 from a mesial line of the disk. This is commonly known as the Camptonccles 

 striation or sculpture, and is common to many recent forms, both ribbed and 

 smooth. It is usually most conspicuous on the submargins, but often plainly 

 visible in the smoother species (such as /'. gronlandicus) over the whole disk. 



The other type of sculpture, which may coexist with any or all of the 

 others, is a product of the minute concentric sculpture due to imbricated 

 incremental lines. In Pccten proper the concentric sculpture is usually simple 

 and sometimes (as in /'. dcxac) almost absent. 



In P. ina.vimus it takes the shape of minute regularly spaced concentric 

 lamella,- on the disk, but on the submargins and part of the cars this sculpture 

 is often crowded and the distal edges of the lamella; more or less concrescent. 

 In L'hlamys, however, the most beautiful and complex surface-sculpture of 

 this sort is found. The lamella; are elevated, and at points corresponding to a 



