﻿344 STROPHITUS 



solid and is perliaps not quite so distinctly and richly painted. 

 As it is a more compressed shell the beaks are naturally not 

 ([uitc so full as they are in D. dyoiuas. 



DIAGEN.^. 



Male and female shells alike; embryos contained in the outer 

 i^ills in short ovisacs, which run crosswise of the branchia\ and 

 arc discharged entire into the water. 



Genus STROPHITUS Rafines(|uc. 1820. 



St ro I'll it lis RaI'Imcsouh. Ann. Gen. Sci. Phys. I'rux., 1820, p. 



3i(). — Oktmann, Ann. Car. Mus., 1912, p. 299. 

 Uniops^is Agassiz. Arch, fiir Nat., I, 1852, p. 49. 



Shell elliptical to rhomboid, inflated, subsolid. i)ointed or 

 biangulate behind, with low posterior ridge, which is some- 

 times double; beaks full, sculpture consisting of a few strong, 

 concentric ridges, which run nearly parallel with the growth 

 lines and curve upward behind ; epidermis rayed or rayless, 

 dull or shining; hinge line incurved in front of the beaks ; teeth 

 rudimentary, a vestigial, compressed tooth in each valve, and 

 sometimes a secondary tooth ; laterals rarely present ; muscle 

 scars shallow. 



Animal having the marsupium occupying the whole outer 

 gills, consisting of short horizontal ovisacs, which run directly 

 across the gills, and are discharged through the outer wall, 

 with the ovules in them ; ovules ten to twenty-five in each ovi- 

 sac, in one or two rows ; inner gills the larger, free in part from 

 the abdominal sac, or wholly united ; mantle generally bordered 

 behind Math square, black spots ; branchial opening with numer- 

 ous papill?e ; anal opening papillose or crenulate. 



Type, And on t a nndtdata Say. 



A small group of North American species, whose shells pre- 

 sent' no very striking characters. l*hey are generally rather 

 thin and present something the appearance of Anodontas, but 

 have in all cases a hinge line incurved in front of the beaks, 

 while in Anodonta it is straight or-evenlv curved in this region. 



