AND ALLIED FORMS. 9 



published in the same "Proceedings," for the month of February, 

 18G3, in a paper entitled a "Systematic arrangement of the Mol- 

 lusks of the family Viviparidfe and others, inhabiting the United 

 States," which has great value as calling attention to the true 

 generic characters of the shell in several groups hitherto little 

 understood or not generally accepted. This naturalist first called 

 attention to the general correlation of size with structure in the 

 families he describes. He agrees with Mr. Tryon in the separa- 

 tion of the Amnicolag as a distinct family, Amnicolidas, to which 

 he gives, however, a much greater extent, by including in it the 

 European Bythinia, and the Bythinella of Moquin-Tandon, which 

 genus he regards as identical with Amnicola proper. He thus 

 follows Moquin-Tandon in approximating these two groups, which 

 have been widely separated by others ; but, apparently recogniz- 

 ing the value of the great difference in the form of their opercula, 

 he proposes to place them in two distinct subfamilies, Bythininae 

 and Amnicolinas.* In the latter group he includes three genei'a, 

 Amnicola, G. & H., Somatogyrus, n. g., and Chilocyclus, n. g. 

 The subgenus Pomaliojjsis of Tryon he rejects as doubtful ; this 

 group, however, in view of the characters of its type P. lapidaria, 

 must be accepted, and Chilocyclus of Gill is synonjmious with it. 



Mr. Gill gives a diagnosis of the proposed family " Amnicolidse" 

 as follows : — 



" Family Amnicolidse (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. BranchisB in a single row, 

 in the form of transverse 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 marcrin of the mantle, on the same side."^ 



The author states that his knowledge of the anatomical charac- 

 ters is chiefly due to Moquin-Tandon, and it will be noticed that 

 this is an exact translation of Moquin-Tandon's description of the 



' Proc. Acad. Nat. Sci. Philad., 1863, p. 34. Tlie presence of cervical 

 lobes in Bythinia is another important point in which it differs from the 

 Amnicolse, etc. 



^ Proc. Acad. Nat. Sci. Philad., 18(J3, p. 35. 



