[NVERTEBRATB PALEONTOLOGY. 577 



nucleus. From Tuloto?na, 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 Anculosd, being found adhering to stones in running water, 

 instead of crawling on muddy bottoms of sluggish streams. 



One or two speeies of this genus exist in such great numbers in Florida 

 that their bleached shells are used in Jacksonville and some other towns to 

 form walks in gardens and ornamental grounds. A few other species also 

 occur there less abundantly, while the genus is well known to range far 

 northward to and beyond the great 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 the Tertiary rocks, at which time it was apparently more 

 abundant in species than at present. We already know a dozen or so 

 extinct species from the Upper Missouri and Rocky Mountain regions. 



Viviparus Leai, M & H 



Plate 44, figs. 6, a, b, c, d. 



Paludina Leai, Meek and Hayilen (1856), Proceed. Acad. Nat. Soi. Philad., VIII, 121. 



Vivipara Leai (18(50), ib., XII, 185. 



Viviparus Leai. Meek and Ha.vden (1863), in Gill's Arrang. Fam. Viviparities, 6. 



Shell conical-subovate ; spire moderately elevated, and pointed at the 

 apex ; volutions six to six and a half, convex, those of the spire regularly 

 rounded; last one composing generally slightly more than half the entire 

 length of the shell, sometimes prominent, or approaching subangular below 

 the middle ; suture well defined ; surface marked by fine lines of growth, 

 which are crossed by traces of extremely fine, nearly obsolete, revolving strise 

 (only visible under a good lens), and at regular intervals by revolving rows 

 of minute shallow punctures; aperture broad-ovate, rather obtusely sub- 

 angular above, and rounded below ; inner lip a little reHexed, but not quite 

 covering the very small, nearly obsolete, umbilical perforation. 



Length, 1.04 inches ; breadth. 0.78 inch ; length of aperture, 0.57 inch ; 

 breadth of same, 0.42 inch ; apical angle convex, varying from 65° to 80°. 



This shell varies considerably in the comparative elevation of its spire 

 at different ages, young individuals being proportionally much more depressed 

 than large specimens. These differences are also more or less distinctly 

 marked sometimes in shells of the same size, and might be regarded of spe- 

 cific importance, were it not for the fact that we find all intermediate gra- 

 73 H 



