226 PROCEEDINGS OF THE ACADEMY OF [1898. 



the middle of the right margin of the lung cavity in some genera 

 (Ariolimax, Aphallarion, Hesperarion), at the right anterior angle 

 in others {Avion, Prophysaon). The rectum lies along the right side, 

 posteriorly, and opens at the breathiug pore (PI. XV, figs. 73, etc., 

 r). Lying in the lung cavity, and often almost filling it, is the kid- 

 ney (PI. XV, figs. 76, 77, 80, etc., k). It is rounded or squarish in 

 shape, thick, and attached by its upper side 10 to the roof of the lung 

 cavity around the heart and toward its posterior side. The kidney 

 appears variously striated or shows branching ducts (as in PI. XV, 

 fig. 74). Its secretion is voided through a slender duct the (second- 

 ary ureter, PI. XV, u. r) emerging posteriorly toward the right 

 side, running parallel to the rectum, and opening near the edge of 

 the breathing pore. The characters of the ureter are insufficiently 

 shown in the figures. Behind the middle of the kidney there is an 

 excavation perforating it, occupied by the heart (PI. XV, a. v.). In 

 some genera both chambers of the heart are exposed below ; in 

 others only the ventricle. Only in Aphallarion are both auricle 

 and ventricle concealed by a thin outer layer of the kidney. We 

 have occasionally observed a portion of the kidney protruded 

 tongue-like from the breathing pore in drowned specimens of large 

 species. The morphology of these organs is not greatly varied 

 among the genera of Arionidse, and need not be farther considered 

 here. In defining the families of Pulmonata, these organs give 

 characters of great value. 



II. CLASSIFICATION. 



The generic characters of slugs in general, and of Arionidse in 

 particular, as given in the standard manuals and faunal monographs, 

 are of the most superficial character. With a single conspicuous, 

 exception, 11 we do not know of any writer who has given evidence of 

 much insight into or comprehension of the meaning of the varied 

 internal structure of slugs, or who has even suggested a phylogenetic 

 arrangement of the genera of Arionidre. 



The complete classification of this and related families cannot be 

 presented here without unduly extending the limits of this paper by 

 including information upon the Old World genera ; but so far as 

 the Arionidae are concerned, the essential outlines may be gathered 



10 Or, as the preparation lies, its lower side. 



11 Need we mention the brilliant author of Die Nachtschnecken der Portu- 

 gesisch-Azorischen Fauna? 



