MYTILID^. 267 



This genus includes several so-called Myalinae, but they have 

 not the thick hinge^plate of the shells of that genus, and species 

 which have been described by Ludwig as belonging to Di-eissensia. 

 The form of the shell is triangular. 



HoPLOMYTiLUS, Sandbergcr, 1850. 



Distr. — Devonian ; Nassau. H. crassus, Sandb. (cxxix, 26). 



Shell equivalve, triangular, p^a-amidal, sides incurved, wider 

 in front, anguhir behind, beaks contiguous ; hinge with an elon- 

 gated septum under the beak, followed by a longitudinal tooth 

 in the right and a corresponding pit in the left valve. 



Pachymytilus, Zittel, 1881. 



Distr. — Jurassic. F. pefasus, d'Orb. (cxxix, 10). 



Shell triangular, very thick and massive, with terminal, promi- 

 nent beaks, and usually smooth surface; anterior to the beak 

 the slope is angularly pinched in, and on the anterior margin, 

 just under the beak, are two fold-like, tubercular teeth ; hinge- 

 margin very thick. 



Subfamily PRASININJE. 



Shell elongated, very inequilateral, smooth, beaks moderately 

 tumid ; hinge with one elongated cardinal tooth in each valve ; 

 ligament external, long, supported by thickened fulcra. 



In Modiolopsis and Phaseolicama no hinge-teeth have been 

 observed. 



HippopoDiUM, Sowb., 1821. 



Distr. — Jurassic ; Europe. H. ponderosum, Sowb. 



Shell oblong, thick, ventricose ; umbones large ; ligament 

 external ; ventral margin sinuated ; hinge with one thick, oblique 

 tooth in each valve, sometimes nearly obsolete ; pallial line 

 simple ; anterior muscular scar deep. This shell appears like a 

 ponderous form of C3''pricardia or Cardita ; it is a characteristic 

 fossil of the English Lias, but only very aged examples have 

 been found. 



Julia, Gould, 1862. 



Syn. — Prasina, Deshayes, 1863. 



Distr. — J. Borbonica, Desh. (cxxviii, 1-3). Isle of Bourbon. 

 J. exquisita, Gould. Sandwich Islands. 



Shell oblong, thick, cordiform, valves closed, margins entire, 

 inequilateral ; lunule deep circular, projecting into the interior 

 of the right valve, left valve in the same place furnished with 

 dentiform tubercles ; hinge-line simple, arched ; ligament 

 external, narrow ; muscular scars two, unequal, subcentral. 



Yery closely allied to, perhaps not generically distinct from, 

 Hippopodium. 



