AND ALLIED FORMS. 11 



often seen the auricles of tliG foot in Amnicola jjorafa, in certain 

 positions, protruded beyond the snout, although their normal po- 

 sition is most certainly in the rear of the snout. Other distinctive 

 marks, not mentioned by Mr. Gill, might be cited for the dis- 

 crimination of the Amnicolaj from the Rissoa;, but none which, in 

 my opinion, are of importance for family distinctions. The deep- 

 water Rissoidse have generally a caudal filament arising from the 

 posterior extremity of the operculigerous lobe, but the shallow- 

 water species are for the most part destitute of this appendage, 

 although so closely allied to the others that Forbes and Hanley 

 have not even generically separated them. The lingual dentition 

 of the Amnicolie is of the same type with that of the Rissoa?, 

 the only essential difference being in the position of the basal 

 denticles of the rhachidian tooth. There may be, indeed, charac- 

 ters remaining to be discovered, which will serve to separate the 

 two groups as distinct families, but certainly none have as yet 

 been brought forward.* 



To conclude the history of the writings of American naturalists 

 on mollusks belonging to the subfamily Hydrobiinae, the paper of 

 Dr. Lewis in the "Proceedings of the Boston Society of Natural 

 History," IX, 161, February, 1863, may be noticed. He has 

 given a short description of the soft parts of Melania isogona, 

 Say, which he refers to Amnicola, as Dr. Lea had already done.? 



My own investigations into the characters of the small North 

 American Gasteropods usually referred to Avinicola, have led me 

 to distinguish among them two distinct subfamilies, which have 



' It may here be remarked that none of the authors quoted ahove have 

 given us valid characters for the distinction of the Amnicola group from 

 the Melaniidse, in which family they are indeed included by Dr. Lea. Mr. 

 Gill, in his Synopsis (loc. cit., p. 33), relies upon the obsolescence of the 

 jaws (an error as shown above), the shape of the aperture, and the size 

 of the shell ; but neither of the last two characters will serve to distinguish 

 our largest Amnicolinse from certain Mudalise and Ancylosse. The same 

 may be said of the continuous peritreme of the aperture of the shell of the 

 Amnicolinae, the character usually relied upon by authors, although this 

 latter has far more value than the others just mentioned. The real dif- 

 ference between the two groups is found in the generative organs, the male 

 in the Melanians being destitute of an external verge. (See a paper " on 

 the structural characters of the so-called Melanians of North America," in 

 Silliman's " American Journal of Science and Arts," [2] XXXVIII, 41.) 



^ Tr. Am. Phil. Soc, IX, 16. 



