TRANSACTIONS OF WAGNER 

 6q2 



^ 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 lira; may be retained 

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

 have the internal lirse of Ainusiiim 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 Pccten and an internally Urate 

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

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

 in a good series of the recent P. hyaliims Poli. While the lirje 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 e.Kternal sculpture to be noted. 

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

 genus CaiHptonectes, 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 Cainptonectes 

 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 P. grdnlandicns) over the whole disk. 



The other type of sculpture, which may coe.xist with any or all of the 

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

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

 and sometimes (as in P. siczac) almost absent. 



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

 lamellee on the disk, but on the submargins and part of the ears this sculpture 

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

 In Chlamys, however, the most beautiful and complex surface-sculpture of 

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



