202 FRESHWATER FAUNA 



now, we turn to the fossils of the Laramie beds, we shall 

 be disappointed when we find that a great number of 

 them are forms such as were already in existence at a 

 much earlier date, the fundamental elements of our 

 existing universal freshwater fauna ; ex. gr., Unio, 

 Anodonta, Cyrena, Sphserium, Pisidium, Limnaea, 

 Planorbis, Physa, Neritina, and Valvata, as well as 

 Melanids, which present the same characters as at 

 present distinguish the Melanids of North America. 



This, however, need not occasion any surprise, for the 

 forms just enumerated had long previously acquired the 

 freshwater habit, and sufficient time had elapsed to 

 ensure their dispersal over widely-extended areas. As 

 an indication of this it may be pointed out that some 

 of these forms are known to occur in approximately 

 contemporaneous deposits of India, where we find, 

 associated with the great lava flows of the Deccan, 

 sediments containing Unio, Physa, Paludina, Yalvata, 

 Limnoea, and Pisidium. That already existing species 

 should take advantage of the opportunity to colonise a 

 new freshwater area is, indeed, only what we might 

 expect. 



But there are other shells present in the Laramie beds, 

 some of which may well be regarded as relics of a marine 

 fauna ; such are several species of oyster, mussel (Volsella), 

 and Anomia. Others which we might at first be apt to 

 regard in the same light appear on further investigation 

 to have originated elsewhere ; such, for instance, are the 

 two important shells known as Pyrgulifera and Goniobasis 

 macilenta (Fig. 70). These are of such extreme interest 

 in another connection, to be considered later, that they 

 demand some little attention. Pyrgulifera humerosa, to 

 give it its full name (Fig. 78), has been identified by 

 Tausch with certain freshwater shells found fossil in 

 beds of about the same age as those of Laramie, but 



