megalodontid^. 207 



Family MEGALODONTID^. 



Shell equi valve, very thick, mostly smooth or finely concen- 

 trically striated ; hinge-plate broad and thick, with two strong- 

 teeth in each valve — sometimes bipartite ; ligament external, 

 supported by thick fulcra ; posterior muscular impression 

 usually upon a prominent ridge. 



Megalodon, J. Sowerby. 



Etym. — Megas^ large ; odos, tooth. 



Syji. — Megalodus, Groldfuss. Tauroceras, Schaf h. 



Distr. — Fossil, 14 sp. Upper Silurian — Devonian; United 

 States, Europe. M. cucullatus, Sowb. (cxxii, 41). 



Shell oblong, smooth or keeled ; ligament external ; hinge- 

 teeth 1*2, thick; laterals 1*1, posterior; anterior adductor im- 

 pression deep, with a raised margin, and a small pedal scar 

 behind it. 



In the tj^pical species the beaks are subspiral, the lateral 

 teeth obscure, and the posterior adductors bounded by prominent 

 ridges. 



[megalomus (Canadensis), Hsillj 1852. Umbones A'ery thick, 

 hinge-teeth rugged, almost obliterated with age ; posterior lateral 

 teeth 1"1 ; no muscular ridges. Upper Silurian; Canada.] 



EUMEGALODON, Giimbcl, 1862. Shell elongated, sometimes 

 longitudinally carinated ; posterior cardinal tooth in the right 

 valve smaller and widely bifid, corresponding tooth in the left 

 valve very prominent and only grooved, posterior lateral tooth 

 well-developed. 31. cucullatus, Sowb. 



NEOMEGALODON, Gilmbcl, 1862. Shell subtrigonal or cordi- 

 form, posterior cardinal teeth bifid in both valves, stronger in 

 the left than in the right one, posterior lateral tooth indistinct 

 or obsolete. Triassic. 31. triqueta, Walfen. 



PACHYMEGALODON, GUmbel, 1862. Posterior cardinal tooth 

 single in the left, double and curved in the right valve, posterior 

 lateral tooth short and distinct ; anterior muscular impression 

 posteriori}^ and inferiorly surrounded by a raised margin. 

 Triassic. 31. chamaeformis, Giimbel. 



Pachyrisma, Morris and Lyeett. 



Etyni. — Pachus, thick ; ereisma, support. 



Type, P. grande, Morris and Lyeett (cxii, 39, 40\ Great 

 Oolite (Bathonian) ; Minchinhampton. 



Shell cordate, with large subspiral beaks ; valves very thick 

 near the umbones, obliquely keeled ; hinge with one thick conical 

 tooth (behind the dental pit, in the right valve), a small lateral 

 tooth close to the deep and oval anterior adductor, and a pos- 

 terior lateral tooth (or muscular lamina ?) ; ligamental plates 

 short and deep. 



