white.] ANNOTATED CATALOGUE. 17 



the shell of Pimia; the pearly layer, which gives such strength to the 

 upper valve, being apparently entirely wanting in the lower. This 

 prismatic layer breaks up into its component prisms with great facility. 

 The characteristics of the under valve of A. micronema, as well as those 

 of the upper valve, show it to be a true Anomia; thus presenting evi- 

 dence of the great antiquity of the genus just as it exists to-day. 



MYTILIDiL. - . 



This family is very sparingly represented in the brackish water strata 

 of North America, and not at all, so far as is now known, in either any 

 existing fresh waters, or in any strata of purely fresh- water origin; yet 

 the family has representatives in some of the strata of all the geological 

 ages, from the Paleozoic to the present time. The only genus of this 

 family which has been recognized among the fossil collections from our 

 brackish water strata is Volsella Scopoli ; and all the examples of it 

 that have been discovered in those strata are apparently referable to 

 the subgenus Brachydontes Swainson. 



Two species have been described from the Laramie Group of Wyo- 

 ming and Colorado, namely Yolsella (Brachydontes) regularis and V. 

 (B.) laticostata White ; * both of which are represented on Plate 13. 



An undescribed form of this genus is also known to exist in the Bear 

 Eiver Laramie beds of Southwestern Wyoming. At least one species, 

 which is closely allied with those just mentioned as coming from the 

 brackish-water strata, is known to exist in the mariue Cretaceous strata 

 of the same region in which those Laramie species occur, and it is not 

 improbable that they are genetically related with each other. 



The genus Dreissend Van Beneden, a living species of which is so 

 common in certain of the rivers of Europe and Western Asia, and which 

 genus is so abundantly and variously represented in the fresh-water 

 Tertiary deposits of Eastern Europe, is not known to be represented in 

 North America by a single species, either living or fossil. Neither is 

 Adacna Eichwald known in North America, either fossil or recent, 

 although so common 'in Eastern Europe and Western Asia, in brack- 

 ish waters and brackish- water formations. 



The genus Mytihis, although it is recognized by Meek in the marine 

 Cretaceous strata of the epoch which immediately preceded the Lara- 

 mie period, seems not to have survived in the brackish waters of that 

 period as did Ostrea, Anomia, and Volsella. 



In the foregoing discussion of the three families Ostreidse, Anomiidse, 

 and Mytilidre, representatives of which are now so abundant upon our 

 marine coasts, it will be seen that especial reference has been had to 

 the faunae of the Laramie and other great groups of strata in the west- 

 ern portion of* the continent. This arises from the fact which has 

 already been stated or alluded to, that in the epochs represented by 

 those groups, the conditions were, in that region, widely extended and 



* An. Rep. U. S. Geol. Sur. Terr, for 1878, Part I, pp. 58, 59, pi. 25, figs. 3 & 4. 

 2 



