428 NON-MARINE FOSSIL MOLLUSC A. 



is now occupied by the North American continent during the epochs 

 that all geologists agree in referring to the Cretaceous period. It can- 

 not be doubted, however, that many representatives of the family really 

 lived during" that period, the remains of most of which are probably for- 

 ever lost, but some of which we may yet hope to discover. The excep- 

 tion that has been referred to is a species, the only known remains of 

 which consist of a few fragments, too imperfect for specific characteri- 

 zation, which were found in the Estuary deposit at Coalville, Utah, 

 which has already been referred to as furnishing Anomiapropatoris, and 

 is yet to be mentioned in connection with certain other species. 



The cause of the apparent paucity of Unione and other non-marine 

 molluscan remains in strata of Cretaceous age, is not, probably, that 

 such inollusca did not then exist in very many places in greater or less 

 abundance ; but it is probably due to the fact that few of the non-marine 

 deposits of those epochs have escaped destruction. 



We come now to the consideration of a geological period, namely, the 

 Laramie (which is also a remarkable period of time in the evolutional 

 history of the Unionidse), in which the physical conditions within the 

 area now occupied by the North American continent were exceedingly 

 favorable to the existence 'and development of non-marine mollnsca. The 

 chief of these conditions was the wide prevalence of brackish and fresh 

 waters during the whole of that period. During the Laramie period 

 there existed a Unione fauna that, for differentiation into a great varietv 

 of subordinate types, is truly remarkable when we remember that it oc- 

 curred at a time so remote. 



It is also a remarkable fact that a large proportion of these types are 

 precisely those which now characterize the peculiar and rich Unione 

 fauna of the Mississippi drainage system. A part only, and apparently 

 an unimportant part, of those subordinate types that existed during 

 the Laramie period appear to have become extinct. 



In tracing the evolutional history of any family of mollusca we should, 

 in a general way at least, expect to find that the simplest forms were 

 the first to appear in the order of time; and although simplicity of form 

 of the shell is not by any means a necessary correlative of simplicity of 

 structure in the mollusk which produced it, we nevertheless naturally 

 inquire whether the simple shells of Anodonta did not precede in geolo- 

 gical time the more complicated shells of Unio. This may or may not 

 have been the case; for the difference in actual zoological rank between 

 the two genera is at best measured only in part by the differences in the 

 shells of each genus. 



If, however, Professor Hall's suggestion is correct, that the two De- 

 vonian forms that have already been referred to, belong to the genus 

 Anodonta; and if Dr. Dawson is correct in referring his Carboniferous 

 genus Naiadites to the Uriioniilae, the evidence seems to be strongly in 

 favor of the opinion that Unio was actually preceded in geological time 

 by Anodonta and other edentulous Unionidse. 



