86 PALEONTOLOGY 



quadrilateral in shape, with long straight hinge with 

 vertical teeth and radial ribs (Fig. 25, a); Pectunculus 

 [Glycimevis], already described (Cret.-Rec.) ; Cucullaa 

 (Dev.-Rec.), shaped like Area, but with long horizontal 

 teeth except under umbo, and a raised edge to the posterior 

 adductor impressions. 



II. DYSODONTA. 



Usually inequi valve ; hetero- or mono-myarian ; fre- 

 quently with anterior and posterior " ears," and a right 

 anterior byssal notch. Cardinal area (if present) and 

 ligament amphidetic. Fixation temporary or permanent, 

 by byssus or cementation. 



i. Pteriacea. Inequivalve (except Pevna and Gev- 

 villia) ; anisomyarian ; internally nacreous ; with ears ; 

 frequently with a byssal notch below the right anterior 

 ear ; hinge-line straight, without teeth or with vestiges of 

 an actinodont type. Silurian to Recent. 



A very large and varied sub-order, in which there is a 

 general tendency (a) to the inequivalve condition, the 

 left valve being the more convex ; (b) to the lengthening 

 out of the hinge-line by the formation of " ears " or 

 " wings " ; (c) to the disappearance of the anterior 

 adductor. The interior is nacreous, the outer layer 

 prismatic; cardinal area and ligament amphidetic; hinge 

 teeth very feebly developed. Ptevia [Avicnla] (Devonian ? 

 to Recent) differs from Pseudomonotis (already described) 

 in its more oblique shape and well-marked left anterior 

 ear (Fig. 25, c, c') ; Pinna (Jur.-Rec.) is acutely triangular 

 in shape; Conocardium (Dev.-Carb.) is equivalve, inflated, 

 the anterior ears forming a sort of tube (Fig. 25, I). 

 These genera are all costate. Perna (Trias.-Rec.) , Gervillia 

 (Trias.-Eoc.), andlnoceramus (Jur.-Cret.) have the ligament 

 partly sunk into the hinge-line, and fixed into a long series 

 of ligament-pits (Fig. 25, d, d')\ these genera are con- 



