PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 101 



The specific name is given in honor of Mr. W. H. Dull, whose iniport- 

 fiut works u})on the molhisca are well known. 



This interesting shell seems to agree with the genns Latia Gray iu 

 all essential characteristics, so far as they are observable upon the 

 specimens that have yet been discovered. The form is Crepidida-lUie, 

 the test thill, and the semilunar septum, well developed; but the "pro- 

 jecting free lamina" upon the right side of the septum, described by Dr. 

 Gray, has not been observed upon our examples. 



Although in form and structure this shell is so much like a Crepidula, 

 its fresh-water associations forbid its reference to the Calyptriidie. I 

 am not entirely satisfied that it ought to be referred to the Ancylidai, 

 but for the present I place it provisionally in that family. 



The fact that this shell is entirely unlike any form that is now known 

 in North America, either living or fossil, gives it peculiar interest. This 

 interest is also largely iiicreased by the fact that the genus to which it 

 is here referred has hitherto been known only in New Zealand or other 

 parts of Oceanica, and only in the living state. 



The molluscan fauna, to which this shell belongs is, as a whole, (juite 

 unlike any other fresh-water fauna of "North America, either living or 

 fossil. The' reason of this difference between the Truckee molluscan 

 fauna and that which now characterizes the Mississippi drainage sys- 

 tem is doubtless that the outlet of the Truckee lake has had no continu- 

 ous connection or identity with the streams that, persisting from Terti- 

 ary time and earlier, have become portions of that system. 



The forms among the Truckee fauna that are most nearly like species 

 now living in North America are the Aneylns and the two species of 

 Splmr'mm just mentioned; and yet the latter present some noteworthy 

 differences from any North American congeneric; form either living or 

 fossil. It is true there is a species of Carinifex in the Pacific drainage 

 waters of California, but its difference from those of the Truckee fauna 

 was regarded by Mr. Meek as of subgeueric importan<5e. The three 

 forms of Melania and the Lithasia of the foregoing list have no true 

 type-representatives, either living or fossil, in North America; and the 

 newly discovered form herein described differs still more widely from 

 any member of any North American fauna. 



The Truckee Group is understood to have quite a large geographical 

 extent in northern Nevada, southwestern Idaho and southeastern Ore- 

 gon, but it has .yet received very little investigation as regards its mol- 

 luscan fauna. The presence in that group of a molluscan fauna so widely 

 differentiated as it is indicated to be by the few species that have hith- 

 erto been discovered encourages the hope that large additions to it will 

 hereafter be made. 



