tHE LAMELLIBRANCHIA 79 



sisting of cardinals without anterior or posterior laterals . 

 Ligament opisthodetic. Ornament variable. Devonian- 

 Recent. 



Under this name, Prof. Douville includes two groups 

 (a) Trigoniacea, with nacreous interior and dental 



formula 3 ^ (Carb.-Recent) ; and (b) Prae astartacea, 



2> 4 b 



with porcellanous interior and dental formula ^' r 



2, 46 



(Dev.-Jur.) 



The Trigoniacea are a small but important group- 

 Schizodus (Carb.-Perm.) and Myophoria (Trias) show a 

 gradual change from smooth and prosogyrous, with tooth 

 2 simple, to the highly-ornamented and opisthogyral 

 Trigonia with tooth 2 bifid. The geological and geo- 

 graphical distribution of this genus have already been 

 mentioned. 



The Pva-astavtacea form a less compact and less familiar 

 group. It includes Cardinia (Fig. 23, d, d') of the Lias 

 (frequently placed in the Naiadacea from its external 

 resemblance to Unio, but non-nacreous and marine), with 

 its old-age (senile, or geronticj modification Hippopodium in 

 which the growth in the direction of thickness leads to a 

 most extraordinary shape. Other genera are Megalodon 

 (Devonian and Rhaetic) and Pachyrisma (Jurassic), large 

 forms in which the hinge is short and high, with big 

 coarse teeth ; and the little Tancvedia of the Jurassic. 



FIG. 23, LAMELLIBRANCHS (continued). 



Pliocene. Interior of left valve. Teeth white, sockets black. (x.) 

 (After S. V. Wood.) f, Diceras minor Deshayes, Upper Jurassic. 

 Anterior view of internal cast. (Natural size.) a, Attachment ; 

 b, c, grooves on cast corresponding to myophoric laminae ; rf, casts of 

 tooth-sockets ; L. K, left, and R.V. } right valve. (After S. P. Wood- 

 ward.) if, Cardiuin parkinsoni J. Sowerby, Pliocene. Interior of valve, 

 (xj.) (After S. V. Wood.) 



