MEGALODONTID#. 207 
Famity MEGALODONTID &. 
Shell equivalve, 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. 
Mercaxopon, J. Sowerby. 
Htym.— Megas, large ; odos, tooth. 
Syn.—Megalodus, Goldfuss. Tauroceras, Schafh. 
Distr.—Fossil, 14 sp. Upper Silurian—Devonian; United 
States, Europe. J. 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 typical species the beaks are subspiral, the lateral 
teeth obscure, and the posterior adductors bounded by prominent 
ridges. 
[MEGALOMUS (Canadensis), Hall, 1852. Umbones very thick, 
hinge-teeth rugged, almost obliterated with age ; posterior lateral 
teeth 1-1; no muscular ridges. Upper Silurian; Canada. | 
EUMEGALODON, Gumbel, 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. J. cucullatus, Sowb. 
NEOMEGALODON, Gumbel, 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. J. 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 
posteriorly and inferiorly surrounded by a raised margin. 
Triassic. I. chamexformis, Gumbel. 
PAcHyYRISMA, Morris and Lycett. 
Etym.— Pachus, thick ; ereisma, support. 
Type, P. grande, Morris and Lycett (exii, 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. 
