NATURAL SCIENCES OF PHILADELPHIA. 35 



lateral, narrow and convex. Tentacles cylindrical, subulate, obtuse, with the 

 eyes on very short peduncles towards their external inferior third. Bran- 

 chiae in three rows, in the form of flattened filaments, slightly subulate, and 

 scarcely dilated towards the inferior third. 



Generative organs on the right side ; verge internal, simple and contained in 

 the tentacle ; female orifice under the border of the mantle on the same side. 



The genus Viviparus, or the Viviparidasofthis synopsis, are the only mollusks 

 which appear to undoubtedly belong together ; of the^ other genera that have 

 been referred by the Adams, Gray and Chenu to the family, Tanalia and Palu- 

 domus are apparently rather related to the Melaniidae, and nearly agree in form 

 of the shell, with Leptoxis of Raf. or Ancylotus of Say, while the structure of 

 the animal, as far as described, is essentially similar to that of the true Melani- 

 ans* and they agree with Viviparus only in possessing an operculum whose 

 elements are concentric, but which is in other respects quite different. The 

 genera Laguncula of Benson, and Rivulina of Lea, are unknown to me. 



All the known types of Vivipari are represented in the United States. 



Tulotoma Hald. 



Viviparus Lam. 



Melantho Bowd. 



Lioplax Trosch. = Haldemania Tryon. 



Family AMNICOLIDJE (Tryon,) Gill. 



Animal oval or elongated, completely retractile within its shell. Foot oval 

 or rounded, generally narrow, and not continued in front of the rostrum. Jaws 

 obsolete. Tentacles cylindrical setaceous, pointed, with the eyes sessile at 

 their postero-external bases. Branchiae in a single row, in the form of trans- 

 verse folds, somewhat dilated at the middle. Generative organs on the right 

 side; verge external, behind the tentacle, bifid and with unequal branches ; 

 female orifice under the margin of the mantle, on the same side. 



The following types belong to this family : 



Amnicola Gld. and Hald. ; Chilocyclus Gill ; Somatogyrus Gill. 



Bythinia Leach. 



The subgenus Bythinella of Moquin-Tandon corresponds to the genus 

 Amnicola of Gould and Haldeman, and consequently should be so treated, al- 

 though its first species apparently belong to the subgenus Pomatiopsis of Tryon ;f 

 the materials now in my possession, or in the Smithsonian Collection, do not 

 enable me to form a satisfactory opinion concerning that group : at present, I 

 am disposed to doubt its distinction from Amnicola. 



In the present paper it is proposed to classify only the Viviparidse. 



The family of Amnicolidae has been recently proposed by Mr. Tryon for the 

 genus Amnicola, but that gentleman has given no diagnosis. It is more nearly 

 allied to the Melaniidae than to the Viviparidas, and the genus Amnicola has 

 been referred to that family by Mr. Lea. 



The material of the Smithsonian Institution, &c, being now in the hands of 

 Mr. Binney, I am unable to arrive at a satisfactory conclusion regarding the limits 

 of our species, and, while expressing my opinions on the classification of our 

 species by reference to those admitted by Binney, except in a single case, I feel 

 somewhat disposed to dissent from him in several instances. As such a differ- 

 ence of opinion would not necessitate any modification of the distribution 

 among groups here proposed, I do not feel at liberty to dissent from him at the 

 present time. J I have seen specimens of almost all the species enumerated. 



* The American Melaniidae, so far as I know, have not a fringed mantle and consequent- 

 ly belong to a different group. 



t The subgenus, as defined by Tryon, is alluded to , the type of that group (A. lapi- 

 daria) may, however, be quite different, and a representative of the family Aciculida;. 



fThe identification of Paludina Elliotti Lea, with P. cyclostomatiformis, is most doubt- 

 ful. I am disposed to believe that, while the latter is a true Melantho, the former ie a 



1863.] 



