REPTILIAN AGE. JURASSIC PERIOD. US 



From Tulotoma these shells are distinguished by being thinner and not nodular; 

 while the animal of the latter genus is said to have more the habits of Anculosa, 

 being found clinging to stones in running water instead of crawling upon the muddy 

 bottoms of sluggish streams and lakes. 



This genus is believed to date back as far as the Jurassic epoch. It also occurs 

 in the Wealden deposits, and was well represented during the deposition of all the 

 members of the Tertiary system, at which time the species seem to have been quite 

 as numerous as at present, if not even more so. We already know some six or 

 seven species from the Lignite Tertiary beds of the Upper Missouri and Rocky 

 Mountain regions. 



Tiviparus Ciilli. 



(Plate V, Fig. 3, a, 6.) 



Shell small, ovate-subglobose ; volutions about four and a half, increasing rather rapidly in size, Tentrioose, 

 rounded, last one larger than all the others ; suture deep and well defined, in consequence of the convexity of the 

 whorls ; umbilical chink very small or closed ; aperture oval, slightly oblique, and nearly equally rounded above 

 and below, there being but a very slight angularity at the inner side above ; peritreme distinctly continuous in 

 adult shells ; surface only marked by small, rather obscure lines of growth. 



Length of the largest specimen seen, 0.47 inch ; breadth, 0.42 inch. 



This seems to be a rather rare species, only three specimens having been found, 

 and none of these are entire. It is most nearly allied to the common recent 

 American species Viviparus lineatus, Valenc, but is much smaller, and differs 

 in having its aperture more nearly equally rounded above and below. There can 

 be no question about its being a typical Viviparus. 



We take pleasure in dedicating this oldest kno'wn American species to our friend 

 Prof Theo. Gill, of Washington City, who first pointed out the true distinguishing 

 characters between this and the allied group Gampeloma, Raf, {== Melmitho of 

 authors). 



Locality and position. — Same as last, where it occurs associated with Neritella 

 Nehrascensis and Lioplacodes veternus. (Type No. 4035.) 



Genus LIOPLACODES, Meek. 



Synon. — Melania (sp.), Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. Deo. 1861, p. 444. 



Lioplacodes, Meek, Smithsonian Check List, Jurassic fossils, 1864, 29 and 40. 

 Etym. — Lioplax. 

 Type. — Melania (_Potadoma) veterna, Meek & Hayden. 



Shell conoid-subovate, rather thick, scarcely perforate; spire prominent; aper- 

 ture obliquely subovate, rather obtusely angular behind, and somewhat narrowly 

 rounded and faintly sinuous in front; peritreme continuous; outer lip inversely 

 sigmoid ; surface with revolving Imes. 



Animal and operculum unknown. 



The shell for the reception of which this genus has been proposed is one of those 

 forms presenting an intermediate appearance between the Melaniidce and Viviparidai, 

 so perplexing where we can know nothing in regard to the nature of the animal or 

 operculum. Its comparatively slender subconical form, and the slight flattening 



