76 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



ularly rounded, or oval, black pigment cells. The interior of the cho- 

 roidea contains a clear, consistent vitreous humor ; but the character of 

 the retina I did not detect at the time. Whether a crystalline lens ex- 

 ists or not I am in doubt ; at the time of making the investigations, in 

 several instances I thought I discerned it very distinctly ; but in other 

 instances, even when larger species were examined, if it existed it es- 

 caped my observation. 



V. CLASSIFICATION. 



WITHOUT suggesting any system of classification, I have already ex- 

 plained (p. 47) that in the descriptive portion of this work I have 

 grouped the genera into families according to the character of the jaw 

 and lingual membrane. I am too well aware that such an arrangement 

 separates genera nearly allied by other, perhaps more important, characters, 1 

 but the same objection may be made to any system thus far proposed. 



The characters on which generic distinction is founded are more satis- 

 factory. These are, the external form of the animal, whether slug-like, 

 as in Limax, or snail-like, as in Helix ; the position of the mantle, anterior, 

 central, or posterior, whether naked, enclosing some form of internal shell, 

 or protected by an external more or less developed shell ; the presence 

 or absence of longitudinal furrows above the margin of the foot, meet- 

 ing over a caudal mucus pore ; the presence or absence of a distinct 

 locomotive disk to the foot ; the position of the external respiratory and 

 generative orifices ; finally, by the absence or presence and character of 

 the jaw, and the character of the lingual dentition. 



When a genus is numerous in species I have, for the sake of conven- 

 ience, adopted sections or subgenera, founded on special features of the 

 shell, such as the absence or presence of internal laminae or tooth-like 

 processes within the aperture. 



In treating the species I have recognized a wide range of variation 

 rather than distinct specific weight in the differences one observes 

 among numerous individuals. 



Guided by these rules, I have grouped our species in the manner 

 shown in the synopsis in Chapter VI. 



1 Thus Veronicella is placed in Holognatha, though it widely differs from the other 

 genera of that family in having contractile, not retractile eye-peduncles, and still more by 

 having separate orifices for the male and female organs of generation. Again Onchidella 

 from the character of its jaw is classed with Patula, etc., though it most widely differs from 

 all the Geophila in having no tentacles, and though its genital system is like Veronicella. 



