FREE INSTITUTE OF SCIENCE 



693 



TERTIARY FAUNA OF FLORIDA 



minute radial line give out little linguiform projections. Often these alternate 

 on adjacent radials and, their distal edges being concrescent, a reticulated 

 cellular sculpture results. This may be still further modified by minute 

 differences of the radials, and finally the distal edges may become completely 

 concrescent, hiding all the cellularity below. By erosion, when still alive, 

 the last stage may be and usually is lost almost completely ; it is more 

 commonly preserved in fossil than in recent specimens. The paleontologist 

 may find, according to the vicissitudes his specimen has undergone, (i) the 

 concrescent surface alluded to ; (2) the reticular cellularity of the lamellae 

 which have lost their upper surface ; (3) the mere tracery of the bases of the 

 lamellje, the walls of the cells being gone; or (4) the surface completely 

 smooth from wear and yet not obviously eroded. Great care is necessary, 

 therefore, not to be misled into describing as different structures which origin- 

 ally were identical. 



The original prototype of Pectcn, judging from the stages of recent 

 shells and the succession of the fossils, was a thin, nearly smooth shell, with a 

 taxodont provinculum and the posterior ears ill-defined. Many sculptured 

 Pectens begin their career in this form. Subsequently the ribbed species with 

 cardinal crura were developed. Next ribbing became obsolete in more seden- 

 tary species. The left valve, being that most in contact with other objects, 

 retained the radial sculpture longest. Species inhabiting soft ooze and depths 

 where motion of the water is feeble and infrequent, or defended by a situs 

 among the arborescent corals or other safe nooks, finally lost the radial 

 sculpture altogether or only retained the internal lirse. The disparity of 

 sculpture between the two valves observable in many deep-water Pectens is 

 perhaps accounted for by the fact above mentioned that the left valve retained 

 the ribs longer than the right valve, and, secondly, that flexibility in the 

 ventral edge of the right valve (incompatible with radial ribbing) became use- 

 ful in excluding the impalpable mud of great depths by its more hermetic 

 sealing of the valves. Consequently, the concentric sculpture of this valve, 

 which is inherent in its mode of growth, alone survived. 



The most modern type of Pecten is doubtless Amusiuvi. That it is derived 

 from a ribbed form is shown by some of the Oligocene species which have 

 (like A. Lyoni Gabb) a nepionic ribbed stage. I believe this group of forms is 

 chiefly sedentary, as the shell and relatively feeble adductor are not suited to 

 rapid motion and the violent shocks involved in this mode of progression. 

 The peculiar hood-shaped form of the distal part of the foot is better suited 



