UPPER CARBONIFEROUS 129 



of the fresh-water habitat of the fauna. On the other hand, externally 

 and internally, Naiadites is extremely like the marine genus Myalina, 

 and Dr. Hind has referred many of our marine Myalinas to Naiadites. 

 In fact, he has even placed the names of some of our American 

 Myalinas which always occur associated with marine faunas in the 

 synonymy of English species of Naiadites which are supposed to be 

 stricdy fresh water. Furthermore, the fauna under consideration 

 is in some instances associated with specimens of Lingula and Aviculi- 

 pecten. The living Lingulas sometimes inhabit brackish waters 

 near the mouths of rivers, but never the fresh waters of lakes and 

 streams, while the living Pectinoids are strictly marine. The fossil 

 Pectinoids in question are small and depauperate examples and belong 

 to a rather peculiar group, that of Aviculipecten whitei. 



This assemblage can hardly be explained as due to the accidental 

 commingling of types having different habitats. If it consisted of 

 fresh-water animals washed out to sea we would expect to find the 

 fresh-water types few and the marine ones numerous, varied, and 

 characteristic. Such is not, however, the case. One would not a 

 priori much expect to find marine animals washed into a fresh-water 

 fauna, and in such an event we would probably look for an entire 

 marine fauna, or, at least, granting that only a few specimens were 

 washed in, that some such invaders would be of the usual marine 

 types. Instead, the alien forms are always limited to one or two 

 peculiar varieties. That abundant and differentiated marine life 

 was always at hand waiting for an opportunity to migrate wherever 

 the conditions became possible seems to be evidenced by the occurrence 

 now and again of marine faunas in close association with beds of 

 coal. On the whole, it seems most reasonable to regard this fauna 

 as a natural assemblage of species selected and modified by a habitat, if 

 not in stricdy marine, at least not in strictly fresh waters. 



There is another reputed occurrence of fresh-water forms in the 

 Carboniferous, reported by Mr. Walcott from the Eureka district, of 

 Nevada. I have examined the fossils in question only casually, and 

 it has been many years since I have seen them at all, but I doubt 

 whether in this instance, any more than the other, the evidence war- 

 rants saying more than that they are possibly non-marine. 



The Upper Carboniferous faunas of the West appear to be better 



