878 QUADRULA 
rough, irregularly, concentrically striate; epidermis reddish- 
brown, wrinkled, rarely feebly rayed; pseudocardinals ragged, 
the anterior of the left valve often compressed and joined to 
the posterior one; muscle scars small and deep; beak cavities: 
moderately deep, compressed; nacre whitish to salmon, dull, 
much thicker in front, sometimes obliquely ribbed. 
Length 64, height 44, diam. 27 mm. 
Length 50, height 38, diam. 26 mm. 
Type locality, Tombigbee River, Moscow, Alabama. 
This species has something of the form of a Pleurobema, 
but it has a higher and different posterior ridge and the beak 
cavities are deeper than in any Pleurobema I know of. There 
are four shells before me, which vary considerably, but they 
agree in essential characters. They are about as much elongat- 
ed as the form I have called chan var. nasuta, but are more 
nearly oval. they have no such beak behind, the posterior ridge 
is lower and the epidermis has a reddish tint. 
QUADRULA RIDDELLIT (Lea). 
Shell short, subquadrate, inflated, sclid, equilateral.or nearly 
so; beaks high and full, turned forward over a lunule, their 
sculpture consisting of numerous corrugated ridges that are 
strongly curved up behind; posterior ridge elevated, carinated 
throughout, ending in a point near the base of the shell; an- 
terior end almost evenly rounded, having a slight oblique trun- 
cation above; base line rounded; posterior end almost squarely 
truncated, the upper part sometimes overhanging a little, with 
an angle behind the ligament; surface unevenly, concentrically 
striate; epidermis greenish-brown; pseudocardinals rough, 
irregular ; laterals short, that in the right valve somewhat dou- 
ble; muscle scars small, deep; beak cavities moderately deep, 
compressed ; nacre bluish-white, thinner and iridescent behind. 
Jlength 35, height 34, diam. 23 mm. 
Length 36, height 34, diam. 20 mm, 
astern ‘Texas; western Louisiana ? 
Type locality, Dallas, Texas. 
