No.120.5. svxoi'sis or mi': x.iuuvs SIMPSON. G73 



'Alaxinodon (Lasmiyona) riridi* RAFIXESQUE, Gout. Monog., 1831, p. 5. 

 ' AltiHinodon (Snlcataria) pupyrucenm RAFINESQUE, Cont. Monog., 1831, p. 5. 

 * Alaxmodon (Snlcataria) badiitm RAFIXESQUE, Gout. Monog., 1831, p. 5. 

 * Alasmodon (Amblamodon) hums RAFIXESQUE. Cont. Mouog., 1831, p. 5, 

 *Margaritana coltimbemis LEA,' Pr. Ac. N. Sci. Phila., XI, 1867, p. 81. 



Genus HEMILASTENA (Agassiz, 1852) Simpson. 

 (Type, Alanmodonta ambiyita Say.)' 2 



Shell small, elongate elliptical, rounded in front and behind, often 

 slightly incurved at the central base; beaks rather sharp, but not full; 

 sculpture consisting of flue, parallel ridges which are looped up in the 

 middle, and open behind; epidermis brownish, rayless; teeth imperfect, 

 a single irregular, compressed tooth in each valve; that of the left 

 under the beak, that of the right in front of it; laterals nearly or quite 

 wanting; anterior end of the shell much thickened; anterior muscle 

 scars united; posterior faint; nacre dull whitish. 



Animal with the marsupinm occupying the whole of the outer branchiae; 

 ovules very large; ovisacs not separated by a sulcus; inner gills the 

 larger in front, free from the abdominal sac part of the way, all four 

 united to the mantle to the posterior end; mantle with a double border; 

 branchial opening large, with many crowded papilla;; anal opening 

 smooth; animal whitish. 



t HEMILASTENA AMBIGUA Say. 



"Alasmodoiita ambiyua SAY, ,11. Ac. N. Sci. Phila., V. 1825, p. 131. * FERUSSAC, 



Guer. Mag., 1835, p. 26. 

 *Margaritana ambiyua KUSTER, Conch. Cab. Uuio. 1862, p. 300, pi. xcix, fig. 7. 



* P.-KTEL, Conch. Sam., Ill, 1890, p. 173. 



* t Unio hildrethianns LEA, Tr. Am. Phil. Soc., V, 1834, p. 36, pi. in, fig. 8; * Obs., 



1, 1834, p. 148, pi. in, fig. 8.* HANLEY, Test. Moll., 1842, p. 196; * Biv. Shells, 

 1843, p. 196, pi. xxm, fig. 38. * CATLOW and REEVE, Conch. Norn., 1845, p. 

 59.* KUSTEH, Conch. Cab. Unio, 1861, p. 206, pi. Lxyin, fig. 8. SOWERBY, 

 Conch. Icon., XVI, 1866, pi. xxxvi, fig. 192.* BAKER, Moll. Chicago, Pt. 1, 

 1898, p. 71, pi. v, fig. 4. 



* Margarita (Unio) Jiildrethianns LEA, Syn., 1836, p. 28; 1838, p. 20. 



* Margaron (filaryaritana) hildrcthianiis LEA, Syn., 1852, p. 43; 1870, p. 69. 



* SiropMtus hildrethiana CONRAD, Pr. Ac. N. Sci. Phila., VI, 1853, p. 263. 



* BapMa hildrethiana H. and A. ADAMS, Gen. Rec. Moll., II, 1857, p. 499. 



* Margaritana hildrethiana B. H. WRIGHT, Check List, 1888. * P/ETEL, Conch. 



Sam., Ill, 1890, p. 173. 



* Alasmodonta dubia FERUSSAC, Gner. Mag., 1835, p. 26. 



Ohio River system; north to Michigan; west to Iowa; soutli to 

 Arkansas; east to Tennessee. 



'Dr. Lea does not mention this in his Synopsis. 



-This species, the only member of the group, differs from Margaritana in the beak 

 sculpture, the thickening of the anterior end of the shell, in the want of muscle 

 scars in the shell cavities, in the color of the animal, and in having the gills united 

 to the mantle posteriorly to their ends, and the shell is not that of a Unio, as it 

 differs in the beak sculpture and the teeth. I regret that Lea's well-known name 

 must be displaced for the obscure one of Say, who undoubtedly had this shell before 

 him when he described his Alasmodonta ambigna. 



Proc. N. M. vol. xxii 43 



