INVERTEBRATE PALAEONTOLOGY. i)43 



ANCYLIDiE. 

 Genus ACROLOXUS, Beck. 



Synon. — Ancylus (sp.), ot'soiue, not of Geoffroy. 



Acrokixus, Beck (1837), Ind., 127. — H. and A. Adams (1855), Genera Recent Moll., II, 263.— 



Chenu (1859), Man. Moll., I, 4-U— Biuney (I860), Land and Fresh-water Shells of N. 

 Am., part II, 1-17. 

 VelleHa (1>Wk Gray, in Turton'a Man. (2d ed., -.'50): and (1842) Synon. Moll. Brit. Mns., 71 

 aod 91. 



Elym. — &npa, apex : tofdf, oblique. 

 Type.— Ancylus lacustris, Linn. 



Shell small, (bin, oval, lira pet-shaped, non-spiral, dextral ; apex usually 

 near the middle, and directed to the left; aperture large; lip thin. 



The little shells of this genus are much like those of Ancylus, with 

 which they agree in their habits, being fresh-water mollusks, found inhabit- 

 ing streams and ponds, where they are usually seen attached to the steins 

 and leaves of aquatic plants. They differ from that genus, however, in being 

 always dextral instead of sinistral, as well as in the details of their lingual 

 dentition. But few existing species are known, and only two of these occur 

 in this country — one in Oregon (A. Nuttallii, Haldeman) and one in Ala- 

 bama (A fi/osus, Conrad, sp.). 



This genus probably dates back to the Cretaceous, or oldest Eocene, 

 and a few species apparently belonging to it have been figured from later 

 Tertiary rocks. 



Acroloxus in i ii 11 1 11 s . 



Plate 44, tig. 10. 



VelMia minuta, Meet and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., 120. 



Ancylus (Acroloxw) minutas, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., 432. 



Shell minute or very small, depressed, extremely thin and fragile ; 

 extremities rounded, the anterior end being usually a little wider than the 

 other. Apex moderately prominent, obtuse, located between the middle and 

 the left posterior side, toward which it is directed. Anterior slope Convex; 

 left posterior slope sometimes slightly concave; surface smooth, or only 

 marked by extremely fine, obscure, concentric stria'. 



Length, 0.09 inch; height, 0.03 inch; breadth, 0.06 inch. 



The specimens of this species yet seen, are very small, and generally 

 distorted or otherwise imperfect. They seem to present some slight differ- 

 ences of form, and may possibly belong to more than one species; but shells 



