268 MYTILID A. 
Moprorarca, Gray, 1840. 
Distr.—2 sp. Falkland and Kerguelen Islands, attached by 
a byssus to floating sea-weed. M. trapezina, Lam. (exxix, 
27,28. 
Shell oval, trapezoidal, ventricose, thin, fragile, covered with 
athin -pidermis; beaks anterior, prominent, contiguous ; anterior 
margil truncate, posterior margin rounded, ventral margin sinu- 
ated and slightly gaping; hinge composed of two little teeth 
in each valve; muscular impressions distinct. 
Mantle-lobes united, pedal opening small, foot with an 
expanded sole, byssiferous, front adductor round. 
PHASEOLICAMA, Valenciennes. Hinge without teeth. M. Magel- 
lanica, Val. (exxix, 29). 
BYSSANODONTA, d’Orb, 1846. Supposed to be related to Ano- 
donta, in the family Unionide, but evidently nearly allied to, if 
not identical with Phaseolicama. It is a minute, perhaps juvenile 
shell, ten millimetres in length, obrotund, thin, not nacreous, 
equivalve, brown, hinge without teeth; foot rudimentary ; fixed 
to rocks by a byssus. B. Paranensis, d’Orb. (exxiv, 93). Near 
Corrientes, Parana Riv., So. Am. 
Mopioropsis, Hall, 1847. 
Syn.—Cypricardites (pt.), Conrad. 
Distr.—45 sp. Silur.; N. Am. JM. faba, Conr. (exxix, 30). 
Shell elongated, very inequilateral, with the beaks anterior, 
subterminal, and close together, thin, hinge toothless, with the 
areal margin more or less straight, and a very slight ligamental 
furrow; ventral margin in front of the middle generally insinu- 
ated, the posterior part of the shell being often much wider and 
higher than the anterior ; anterior muscular impression distinct 
and large. 
Hipromya, Salter. 
Distr.—Fossil, 1 sp. Devonian. 
Shell gibbous, with anterior, inflated close beaks,a long ecar- 
dinal edge; anterior edge short, rounded, and separated by a 
strong sinus from the inflated posterior ridge and slope. 
Myoconcna, J. Sowerby, 1824. 
Etym.— Mya, mussel; concha, shell. 
Syn.—Modiolina, Muller. 
Distr.—Fossil, 26 sp. Permian—Miocene (d’Orb.) ; Europe. 
M. crassa, Sowb. (exxix, 31). M. angulata, d’Orb. (exxix, 32. 
Shell oblong, thick, with nearly terminal depressed umbones ; 
ligament external, supported by long, narrow, appressed plates ; 
hinge thick, with an oblique tooth in the right valve; anterior 
muscular impression round and deep, with a small pedal scar 
behind it ; posterior impression large, single ; pallial line simple. 
cal Peer 
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