1288 DiPi.onoN 



moderate ; posterior slope with a faint, radial depression ; sur- 

 face roughly, concentrically striate; epidermis greenish-brown; 

 pseudocardinals and laterals lamellar; nacre purplish. 



Length probably t^2, height 30 mm. 



Rio de la Plata. 

 Afya variabilis INIatox, Tr. Linn. Soc. Lond., X, 181 1, p. 327, 



pi. XXIV, figs. 4-7. — Wood, Ind. Test., 1825, p. 13, pi. 11^ 



fig. 38 a: rev. ed., 1856, p. 17, pi. iii, fig. 38. 

 Unio variabilis d'C)Rr.](;NY, Voy. Am. Mer., 1843, p. 604, pi. 



i,xxi, figs. 1-3. — SowERi'.v, Conch. Icon., XVL 1868, pL 



i^xxiv, fig. 381. 

 Margarita (Uniu) I'ariabilis Lka, Syn.. 1836, p. t,^\ 1838, p. 2^, 

 Margaron (Unio) variabilis hEA, Syn., 1852, p. 35; 1870, p. 56. 

 Diplodon variabilis Simpson, Syn., 1000, p. 888. 

 Mytilus matoniana d'OKBiCNV, Guer. Mag., 1835, P- 35- 

 Unio matonianiis P.iiTET,, Conch. Sam., IH, 1890, p. 158. 

 f Unio membranacea Hanley, Biv. Shells, 1843, P- 202. pi. 



XXII, fig. 6. 

 Unio rnembranaccns Piiiltppi, Conch., IH, 1848, p. 80, pi. v,. 



fig. 4. — KusTiCR, Conch. Cab. L^nio. 1862, p. 284, pi. xcv, 



%• 5- 



Maton gives four figures, one of which is an interior and 

 another is of a young shell, which I am sure belongs to a dif- 

 ferent species. Plis shell difi"ers from bnlloides, being more 

 nearly orbicular, less sharply truncated posteriorly, in having 

 apparently a lower posterior ridge with a feebler excavation 

 above it and in the color of the nacre, which, he says, is pur- 

 plish. 



Subgenus PTyridet.t^a Swainson, 1840. 



Hy ridel la Swainson, Treat, on Mai., 1840, p. 380. 



Beaks rather low, sculpture consisting of curved, generally 

 nodulous ridges, which approach below but usually have a 

 smooth area of shell between them; surface sulcate or some- 

 times corrugated and nodulous ; epidermis rayless ; teeth rather 

 delicate, compressed, often somewhat rudimentary. 



Animal having the embryos occupying the inner gills for 

 the most p.nrt. which are united for their entire length to the- 



