WHITE.] ANNOTATED CATALOGUE. 441 



we find in the strata of the Laramie Group, but also the belief that the 

 brackish water Cyrenida3 which existed contemporaneusly with them 

 in the Laramie period are not thus ancestrally related to the living 

 species of Cyrena and Corbicula of North America, but that the latter 

 have come down by some other lines of descent which are not yet known. 



CORBULIDJG. 



The genus Corbula is represented by several species in the Cretaceous 

 strata of North America 5 but all the species that are yet known from 

 strata of that period are found associated with marine forms] and, as 

 they no doubt lived in marine water*, they do not fall within the scope 

 of this article. Three or four species are known to have lived during 

 the Laramie period, the faunal associates of which indicate a brackish 

 water habitat 5 and as no other fossil species of this genus have been 

 discovered under such circumstances as to make it probable that they 

 lived in other than marine waters, we shall have only those Laramie 

 forms to discuss at the present time. 



Nine species of Corbula have been described and named by different 

 authors, from strata which have been studied at different localities, 

 all of which strata are now referred to the Laramie Group; but at pres- 

 ent I am disposed to regard less than half of them as distinct species, 

 the remainder being regarded as at least no more than varieties of those 

 species, respectively. 



Mr. Meek regarded all the Laramie species of Corbula as not belong- 

 ing to the typical section of the genus; and he referred one of them 

 to the proposed subgenus Anisorhynchus of Conrad, and the others 

 to Pachyodon Gabb. In this article, however, I shall treat all the spe- 

 cies herein discussed as true Corbula, without expressing an opinion as 

 to the sections of the genus proposed by Conrad and Gabb; but it is 

 proper to call attention to the fact that tho first two mentioned of the 

 following species present a facies or type which is quite different from 

 that of any living North American species of Corbula. Indeed, we may 

 regard these Laramie forms as representing extinct lines of descent, for 

 it is practically certain that the line of descent of none of the living 

 species of Corbula has come down through any of those of the Laramie 

 period as represented by the strata which we now know as the Laramie 

 Group. 



The reasons for this opinion are expressed in the remarks that have 

 been made in relation to the CyrenidiB of the Laramie Group, and the 

 subject is further discussed in the remarks which close this article. 



Since the Bear Eiver beds are hypothetically regarded as the earlier 

 portion of the Laramie Group, the first of the species of Corbula to be 

 mentioned is C.pyriformis Meek,* which has been found only in those 

 beds, and their equivalents in Southwestern Wyoming and the adjacent 



*See Simpson's Rep. Great Basin Utah, p. 361, pi. v, figs. 9 and 10. Also U. S. 

 Geol. Sur. 40th Parallel, vol. iv, p. 170, pi. xvii, fig. 2. 



