CLASS I 



PELECYPODA 



459 



becoming obsolete in the adult and replaced by the typically isodont development of the 



crura ; otherivise as in the Pectinidae. Trias to Recent. 



Plicatula Lain. {Harpax Park.) (Fig. 716). Shell 

 compressed, with coarse radial, often divaricate 

 ribbing, a small area, and long shallow crenulate 

 crural teeth, diverging at a sharp angle. Trias to 

 Recent ; maximum in Jura and Cretaceous. 



Spondylus Linn. (Figs. 717, 718). Shell inflated, 

 with radial, often spiny or foliaceous sculiature ; 



Fig. 716. 



Plicatula pcctlnoldes Lam. Middle Lias ; 

 Nancy, France. 



^^^ 









Fig. 717. 



Sjmndylus sjrinosus (Sow.). Planerkalk ; 

 Strehlen, near Dresden. 2/3. 



Fig. 718. 



Spondylus tenuispina Sandb. Oligoceno ; Waldbiickellieim, near 

 Kreuznach, Prussia. Vi- 



attached valve with a consjiicuous area ; crural teetli heavy, short, smooth. Jura, 

 to Recent ; maximum from the Tertiary onward. 



(?) Pachypteria de Koninck. Carboniferous. P. nobilissiina (<Ie Koninck). Pro- 

 spondylus Zimmerm. Permian and Trias. Philippiella Waagen. Alpine Trias. 



Family 35. Dimyidae Dall. 



Shell inequivalve, irregular, closed, auricles not differentiated, Ostreiform, dimyarian, 

 sessile ; shell substance sub-nacreous and 

 fibrous ; area amphidetic, obscure ; liga- 

 ment obsolete, resilium alivincular, in- 

 ternal; hinge armature taxodont, obsolete; 

 crural development feeble ; gills fili- 

 branchiate, the direct filaments not 

 reflected ; foot and byssus absent ; an- 

 terior adductor distinct, small ; posterior 

 duplex, larger. Trias to Recent. 

 - Dimya Rouault {Dimyodon Mun.- 

 Chalm.) (Fig. 719). The Recent forms inhabit deep water 



Fig. 719. 



Dimya deshayesiana Rouault. Eocene ; Pyrenees. Inner 

 and outer views of riglit valve, ^2 (after Rouault). 



Family 36. Limidae d'Orliigny. 



Shell equivalve, a^mculate, gaping, Pectiniform, monomyarian ; shell substance 

 fibrous, ivith minute tubules, not nacreous or prismatic ; hinge edentulous, or vjith traces 

 of taxodont armature; area amphidetic, equal in both valves; ligament alivincular, 



