MYTILIDA. 267 
This genus includes several so-called Myalinz, 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 Dreissensia. 
The form of the shell is triangular. 
Hoptomytitus, Sandberger, 1850. 
Distr.—Devonian; Nassau. A. crassus, Sandb. (cxxix, 26). 
Shell equivalve, triangular, pyramidal, sides incurved, wider 
in front, angular 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. 
PacuymMyTitus, Zittel, 1881. 
Disir.—Jurassic. P. pe/asus, d’Orb. (exxix, 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 PRASININZ. 
Shell elongated, very inequilateral, smooth, beaks moderately 
tumid ; hinge with one elongated caidinal tooth in each valve; 
ligament external, long, supported by thickened fulcra. 
In Modiolopsis and Phaseolicama no hinge-teeth have been 
observed. 
Hiproropium, Sowb., 1821. 
Distr.—Jurassic; Europe. H. ponderosum, Sowb. 
Shell oblong, thick, ventricose; umbones large; ligament 
external; ventral margin sinuated ; hinge with one thick, obiique 
tooth in each valve, sometimes nearly obsolete; pallial line 
simple; anterior muscular scar deep. ‘This shell appears like a 
ponderous form of Cypricardia 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. (exxviii, 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. 
Very closely allied to, perhaps not generically distinct from, 
Hippopodium. 
