422 NON-MARINE FOSSIL MOLLUSCA. 



apparently the only immediate associate of Ostrea, the same general 

 remarks may be made concerning the Anomiidse that have just been 

 made concerning the Ostreidss so far as they are applicable to the sub- 

 ject of this article. The earliest known North American species of Ano- 

 mia have been found in Cretaceous strata, the greater part of them having 

 by the different authors who have described tbem been reported as as- 

 sociated with forms that must be regarded as of marine origin, but some 

 of them are known to have existed in the estuaries that indented the 

 sea coasts of the Cretaceous period. 



One estuary species, A. propatoris, White, was discovered by Mr. 

 Meek in an interesting estuary deposit of Cretaceous age at Coalville, 

 Northern Utah, where it was found associated with Cyrena, Unio, Val- 

 vata, Melampus? Physa, and also with some marine forms. It is rep- 

 resented on Plate 5. In the marine Cretaceous strata of the same 

 neighborhood some imperfect examples of Anomia have been found 

 which seem to be specifically identical with Anomia propatoris. If this 

 identification is correct it seems to prove that the species in question 

 ranged from marine to brackish waters. This supposition is a plausible 

 one, because certain living species of mollusks are known to have a 

 similar range of habitat. 



Anomia propatoris is very closely like some of the various forms of 

 A. micronema presently to be mentioned, and the former not improbably 

 represents the latter species ancestrally.* 



Two other species of Anomia only are known, which come within 

 the scope of this article, both of which are found in the strata of the 

 Laramie Group in Colorado and Wyoming 5 although it is by no means 

 unlikely that other species existed in the brackish waters of all the 

 epochs that have passed since the family was first established.. These 

 two species are A. micronema and A. gryphorhynchus, Meek. They 

 are both represented on Plate 12. Both are from the Laramie Group, 

 and although in the same neighborhood they are seldom found associ- 

 ated in one and the same layer. 



It has been the subject of frequent remark that not a single example 

 of the under valve of either of the three species of Anomia herein no- 

 ticed has ever been discovered, although hundreds of examples of the 

 upper valves of at least two of the species has been obtained, at many 

 different localities, in a good state of preservation. I was lately so 

 fortunate however as to find in the Laramie strata of Northeastern 

 Colorado several examples of the under valve of A. micronema, one of 

 which is illustrated by Fig. 11, on Plate 12. That the under, or byssus- 

 bearing, valves of A. micronema at least have been so generally de- 

 stroyed is due to the fact, first, of their extreme thinness, and, secondly, 

 to the fact that, with the exception of a thin, porcelanous layer in the 

 middle portion, the whole valve is composed of a prismatic layer, like 



*See remarks in An. Rep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 14, pi. 12, fig. 15. 



