100 PROCEEDINGS OF THE MALACOLOGICAl SOCIETY. 



invaded throughout by the body-cavity. (2) The original concen- 

 tration of the retractor muscles posteriorly, gives way to the tendency 

 toward separated, independent insertions. (3) The shell becomes 

 flat, buried, and finally reduced to incoherent granules. (4) The 

 penis degenerates, and its function is assumed by other organs. 



The shortening of the kidney I regard as of less significance, since 

 it is a mechanical result of the shortened lung by the reduction of 

 shell and mantle. 



The separation of the ocular retractor muscles is probably traceable 

 to a mechanical cause ; the degenerate shell no longer affords any 

 better fulcrum than the rest of the integument, and the oblic[ue 

 stress on these muscles tends to pull them apart. 



The degeneration of the penis, while a marked feature in Arionidse, 

 is not peculiar to the family, occurring also in Helicidse and elsewhere. 

 This modification, it is demonstrated, occurred independently in several 

 genera of the family. 



The ground having been thus cleared, so far as at present possible, 

 it remains to construct a classification which shall express the 

 ascertained relationships and phylogeny of the genera. To avoid 

 repetition, the characters of the genera of Binneyinse and Ario- 

 liraacinfe are here much abridged, because they have been given in full 

 elsewhere ; ^ the Arioninse, comprising all known old-world genera 

 of the family, being more fully exploited. 



Synopsis of Subfamily and Generic Chaeacteks. 



A. Retractor muscles of tentacles converging backwards, their pos- 

 terior terminations with that of the pharyngeal retractor, 

 contiguous, inserted in a group at the posterior edge of the 

 diaphragm mesially. (PI. YII, Fig. 4.) 

 b. Tail solid, the viscera crowded forward and elevated into 

 a dorsal hump ; shell partly or wholly exposed ; penis 

 present, with retractor muscle ; ovotestis posterior in the 

 body-cavity ; a retensor muscle present. 



Subfamily Binneyin^. 

 c. Sole tripartite ; shell Vitrina-sha-ped, wholly exposed, with 



differentiated nepionic whorl. Binneya. 



c'. Sole undivided ; shell a flat or convex, non-spiral plate, 

 partially covered, etc. Hempliillia. 



V. True slugs, the foot excavated throughout, body-cavity and 

 viscera extending to tail ; shell flat, wholly buried ; ovo- 

 testis anterior to posterior loop of intestine ; intestine long, 

 spirally twisted; main cusps of the side teeth lengthened. 



Subfamily Ariolimacin^. 

 c. Retensor muscle present ; ovotestis anterior to stomach ; no 

 appendicula on genitalia. 



' Pilstry & Vanatta, " Eevision of the American Slugs" : Proc. Acad. Nat. Sci. 

 Philadelphia, 1898, p. 227. 



