64 NON-MARINE FOSSIL MOLLUSCA. 



types ; but two or three of the revolving striae upou the smaller volutions of 

 these examples are much more prominent than in the case of typical ex- 

 amples. I at present, however, regard these as only varieties of a very 

 variable species. Examples of this variety are illustrated with the 

 others on Plate 26. 



The collection made by Mr. Allen also contains Vivipams trochiformis 

 and V. retusus and V. leai Meek & Hayden, and Unio senectas White. 



VALVATID^;. 



The Valvatidae like the Rissoidae are inconspicuous among the fossil 

 non-marine faunas of North America, but the family which is essentially 

 a fresh-water one, seems to have become established at least as early as 

 Mesozoic time. Indeed, although the Valvalidce are among the more 

 highly organized of the fresh-water inollusca, they seem to have co- 

 existed with the earliest fresh-water gill-bearing mollusks that are yet 

 known to us. 



Among the fresh-water Jurassic fossils that were obtained by Meek 

 & Hayden from the vicinity of the Black Hills, and which have been 

 already several times mentioned on previous pages, is a form to which 

 those authors gave the name of Yalvata seabrida,* and which is the 

 earliest known species of that genus in North American strata. It is 

 illustrated on Plate 3, by a copy of Meek's figure. 



The next known species in the order of time is Y. nana Meek,f which 

 was obtained by him from the Cretaceous estuary deposit at Coalville, 

 Utah, which, has before been mentioned. It is illustrated on Plate 5. 



Prom the Judith Eiver Laramie beds of the Upper Missouri River 

 region, Meek & Hayden described a form under the name of Yalvata ? 

 montanaensis ; and from the Fort Union beds of that region, two others, 

 under the names, respectively, of V. subumbilicata and V. parvula.% The 

 latter is probably only a variety of the former. All three of the last 

 named forms are illustrated on Plate 27 by copies of Meek's figures. 



For the purpose of presenting the subject of the foregoing pages 

 synoptically, the following table has been prepared. It is intended as 

 a list of all the known species of the fossil non-marine inollusca of North 

 America; and is also intended to show the geological position of each 

 species, and consequently the present known range in geological time, 

 of the families and genera to which they belong. The names of the di- 

 visions of geological time used in this table will be found explained ou 

 page 8 et seq. 



In making up the following list, preoccupied names, and generic names 

 which have been wrongly used, are omitted, but such as are regarded as 



♦Paleontology of the Upper Missouri, p. 113, pi. iv, fig 2. 

 + See An. Rep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 33, pi. 12, fig. 17. 

 { For descriptions and figures of these three forms see U. S. Geol. Sur. Terr., vol. ix, 

 pp. 590-592, pi. 42, and woodcuts. 



