38 RESEARCHES LTON THE HYDROBIIN^ 



position of this genus still remains in doubt, although, notwith- 

 standing the "vascular sac" and its amphibious habits, it seems 

 to nie most probable that it will prove to be branchiferous. For, 

 as Mr. Blanford has pointed out, its characters exclude it from 

 all known families of pulmoniferous gasteropods. 



[Since the above remarks were written, I have received, through 

 the kindness of Mr. W. H. Dall, a specimen of Cremnobates syn- 

 hydrsensis, which, fortunately, contained the dried animal. I 

 find that its dentition is entirely that of the Littorinidse.] 



ni SYSTEMATIC DIAGNOSES OF THE GENERA OF 

 HYDROBIINiE. 



The subfamily Poraatiopsinse, as far as is certainly known, con- 

 tains but one genus, so that its further illustration is at present 

 unnecessai'y. With the Hydrobiinae it is far otherwise, no less than 

 fourteen genera being indicated by the considerable differences to 

 be observed among the species of this subfamily, which are very 

 numerous in all parts of the world. In the recent monographs 

 of von Frauenfeld,* one hundred and twenty-five species are enu- 

 merated, a few of which, however, belong to the allied subfamilies 

 Kissoinse and Pomatiopsinse. But the German naturalist has 

 naturally overlooked some described species on account of their 

 having been placed in wrong genera ; and there aro» numerous 

 undescribed species in the collections of American conchologists, 

 particularly in that of Mr. Tryon, which would make the total 

 number of species nearly two hundred. 



The generic place of very many of these species, known as yet 

 by the shell only, must remain undetermined until the soft parts 

 and the lingual dentition are examined. Certain genera^as Steno- 

 Ihyra, Tricula, Pyryula, and Tryonia, are indeed easily recog- 

 nized by the shell alone ; but no characters are found in it which 

 will enable us to distinguish certain Littorinellaj and Hydrobise 



' See Verhandl. der k.-k. zool.-bot. Gesellschaft in Wien, XIi; (1862,) 

 pp. 1158-llCO, and XIII, (1863,) pp. 193-210; 1017-1032. This author 

 distributes the species of the subfamily among five genera, — Xematura, 

 Lithofjiyphns, PdlndinfUa, TTjjdroJiin, and Amnicola, but he gives no charac- 

 ters for the distiTiction of the last three genera, and confesses the difficulty 

 of doing so until the soft parts are better known. 



