764 PROCEEDINGS OF THE NATIONAL MUSEUM. 



*Unio striatissimus ANTHONY, Am. Jl. Couch., I, p. 156, 1865, pi. xn, fig. 1. 

 *SOWERBY, Conch. Icon., XVI, 1868, pi. LXXXVI, fig. 460. *P^ETEL, Conch. 

 Sam., Ill, 1890, p. 168. 



Opper Tennessee River drainage. 



Section PLETHOBASUS Simpson, 1900. 

 (Type, Unio cwopua Green.) 



Shell large, irregularly oval, inflated, solid, somewhat suddenly swollen 

 at the posterior base; posterior ridge low and rounded; beaks rather 

 high near the anterior end, havirfg a few strong ridges which are 

 curved upward behind; a row of low, irregular tubercles extends from 

 near the beaks to the post basal part of the valves; epidermis tawny 

 brown in the younger shell, becoming darker with age; hinge plate 

 solid, not flattened; pseudocardinals triangular, rough; cavity of the 

 beaks not deep; front part of the shell very heavy; hinder part much 

 thinner. 



Animal having the mantle somewhat toothed posteriorly; branchial 

 and anal openings large, the latter smooth or with very faint papillae; 

 inner gills the larger, rounded below, free nearly or quite their whole 

 length from the abdominal sac; marsupium occupying the entire outer 

 gills. 1 



tPLEUROBEMA ^ESOPUS Green. 



* Unio cesopus GREEN, Cont. Mac. Lyceum, I, No. 2, 1827, p. 46, fig. 3. * HANLEY, 



Test. Moll., 1842, p. 181; *Biv. Shells, 1843, p. 181, pi. xxiv, fig. 7.* CAT- 

 LOW and REEVE, Conch. Nom., 1845, p. 551. *H. and A. ADAMS, Gen. Rec. 

 Moll., II, 1857, p. 496.* CALKINS, Proc. Ottawa Acad. N. ScL, 1874, p. 41. 

 *CALL, Jl. Cinti. Soc. N. H., XVIII, 1896, p. 157, pi. vi. 



* Margarita (Unio) cesopus LEA, Syn., 1836, p. 17; 1838, p. 16. 



*Margaron ( Unio) (esopus LEA, Syn., 1852, p. 23; 1870, p. 34. 



*Unio cyphia CONRAD, New F. W. Shells, 1834, p 68. *FERUSSAC, Guer. Mag., 

 1835, p. 27. 



*Unio cyphius SAY, Am. Conch., VI, 1834. * CONRAD, Pr. Ac. N. Sci. Phila.,VI, 

 1853, p. 248.* REEVE, Conch. Icon., XVI, 1864, pi. vm, fig. 28. 



* Unio cyphias var. assopus P^TEL, Conch. Sam., Ill, 1890, p. 150. 



* Unio scyphius KUSTER, Conch. Cab. Unio, 1861, p. 181, pi. LVII, fig. 2. 



Ohio, Cumberland, and Tennessee river systems; west to Missouri 

 and Minnesota; Alabama River, according to Call. 



1 1 am not positive as to the generic position of the two following species, but 1 am 

 inclined to place them in Pleurobema. No other living species of this genus has any 

 development of tubercles, but a great number of fossil forms from the Tertiary of 

 eastern Europe which seem to be most like Pleurolema are decidedly pustulous 

 The heavy, inflated, high beaks and the comparatively shallow beak cavities are 

 characters found in all the species of this genus. In the single gravid specimen I 

 examined (one out of a large number from different localities) the outer gills were 

 not filled very full of ova, though they were found throughout their extent, while 

 the most careful search did not disclose any in the inner gills. 



