268 MYTILID-S!. 



MoDiOLARCA, Gray, 1840. 



Distr. — 2 sp. Falkland and Kerguelen Islands, attached by 

 a byssus to floating sea-weed. M. trapezina, Lam. (cxxix, 

 27,28\ 



Shell OA^al, trapezoidal, ventricose, thin, fragile, covered with 

 a thin epidermis ; beaks anterior, prominent, contiguous ; anterior 

 margin 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, Yal. (cxxix, 29). 



BYSSANODONTA, d'Orb, 1846. Supposed to be related to Ano- 

 donta. in the family Unionidge, 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. (cxxiv, 93). Near 

 Corrientes, Parana Riv., So. Am. 



MoDTOLOPSTS, Hall, 184t. 



Syn. — Cypricardites (pt.), Conrad. 



Distr. — 45 sp. Silur. ; N. Am. iff. /a6a, Conr. (cxxix, 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 yqyj 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. 



HippoMYA, Salter. 



Distr. — Fossil, 1 sp. Devonian. 



Shell gibbous, with anterior, inflated close beaks, a long car- 

 dinal edge ; anterior edge short, rounded, and separated by a 

 strong sinus from the inflated posterior ridge and slope. 



Myoconcha, J. Sowerby, 1824. 



Etym. — Mya.1 mussel ; concha., shell. 



Syn. — Modiolina, Miiller. 



Distr. — Fossil, 26 sp. Permian — Miocene (d'Orb.) ; Europe. 

 M. crassa, Sowb. (cxxix, 31). M. angulata, d'Orb. (cxxix, 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. 



