INVERTEBRATE PALEONTOLOGY. 561 



occur in the streams of the Southern and Western Stales, northward to the 

 region of the Great Lakes, while a few are found in the streams of the Pacific 

 shores, northward into Oregon. It is a strictly American group, none of the 

 species being known to occur in the European or Oriental districts. 



Professor Haldeman first noticed the fact that all of the American types 

 of so-called Melanians, including this and several allied genera, differ from 

 the true Melaniidce of Eastern countries, in having the mantle-margin not 

 fringed. This character was also noticed at a later date by Professor Gill, 

 who proposed to separate the American genera, as a distinct subfamily of 

 Melanians, under the name Ceriphasince* Professor Haldeman also proposed 

 in 1863 to separate the American types, but as a distinct family, under the 

 name Strepomatidce, which has been adopted by Mr. Tryon in his Mono- 

 graph, already mentioned. 



It is often very difficult to separate the species of this genus from each 

 other, as well as to decide to which one of several allied genera particular 

 species should be referred, even among recent types, both the shell and 

 animal of which are well known. Hence it may readily be understood that 

 the classification of battered, worn, and distorted fossil shells of this kind 

 must long remain merely provisional. In these cases, we can only adopt such 

 conclusions as may, with the material at hand for study, seem to be most 

 probably correct, feeling at the same time perfectly conscious that additional 

 collections may show forms that seem to be distinct to be mere varieties of 

 one species. 



We have pretty good evidence that the genus Goniobasis dates back as 

 far as the latest Cretaceous period in the Rocky Mountain region, where it 

 also occurs in Tertiary beds of different ages. In this elevated interior part 

 of the country, these fossil shells often occur in very great numbers of individ- 

 uals, though, so far as yet known, the species would seem not to have been so 

 numerous as those now living in the streams of the Southern and Western 

 States. It is a little remarkable that no existing species have yet, so far as 

 I am informed, been found in the streams of this distant internal region, 

 where the remains of fossil species are so abundant. 



* Proceed. Acad. Nat. Sci. Philad., XV, 33, 1S03. 

 71 H 



