INVERTEBRATE PALAEONTOLOGY. 285 



PULMONATA. 



SIPHONARIIDiE. 

 Genus ANISOMYON, M. & H. 



Synoit. — Patella, Acnum, Helcion, &c. (sp.), of palaeontologists. 



Anisomyon, Meek ami Hayden (1860), Aui. Jour. Sci. and Arts, XXIX (2d ser.), 35.— Gabb (18C1), 

 Synop. Moll. Cret. Form., 38; (1861) Palaeont. Cal., I, 142; and (1869) ib., II, 231.— 

 Meek (1804), Smithsonian Check-List N. Am. Cret. Fossils, 17.— Stoliczka (1868), 

 Palasont. Indica, II, 325.— Dall (1870), Am. Jour. Conch., VI, 39. 



Etym.—avicoc, unequal ; pvi>v, muscle. 



Type. — Helcion patelliformis, Meek and Hayden. 



Shell \ery thin, patelliform, obliquely conical, or bonnet-shaped ; base 

 oval or circular, with .simple margins; surface nearly smooth, or only marked 

 by obscure lines of growth, crossed on some species by fine, faintly-marked, 

 radiating striae, and sometimes a few obscure posterior ridges ; summit more 

 or less elevated, located between the middle and the anterior end, or some- 

 times nearly central ; immediate apex very small, abruptly curved backward, 

 but not^ spiral; interior without a siphonal groove, projecting lamina, or 

 other appendage ; horse-shoe-shaped muscular scar enlarged at the anterior 

 extremities, with the open part directed toward the shorter end of the shell, 

 becoming abruptly attenuate, or broken into a few small spots on the right 

 posterior side ; anterior extremities connected by a linear impression, usually 

 passing across just in front of the summit. 



On the left side of (he shell, the anterior end of the muscular impres- 

 sion is generally less enlarged than on the right, but sometimes extends 

 slightly farther forward. Posteriorly, it passes around in the form of a band 

 to the middle of the slope behind (figs. 5, c and/ pi. 18), where it is abruptly 

 enlarged and curves upward From this point to the larger anterior extremity 

 on the right side, there is usually only a slender line, or row of spots. Some- 

 times this slender line seems to be quite entire, while in other specimens, 

 even of the same species, it is broken into a few small spots or scars, as seen 

 in tig 5, f 7 of pi. 18 ; and in still other examples it seems to be entirely obso- 

 lete so as to leave the enlarged anterior extremity on the right quite isolated, 

 as seen in fig. 5, c, of the same plate. 



In most instances, the specimens, as broken or weathered from the 

 matrix, have lost the small reflexed apex, as seen in our figures 7 «, and 9 

 b, r, of pi. 18; and, in this condition, its former existence would scarcely be 



