XIV MORPHOLOGY OF TEETH. 



pi. 51, figs. 1, 2, and pi. 48, all figs.). This modification is especially 

 characteristic of one of the primary divisions of Helices, but occurs 

 also on a few isolated genera, such as Allognathus, of other phyla. 



Radulw of arboreal snails. Data presented in the systematic por- 

 tion of this volume establish the fact that arboreal snails always 

 assume teeth with broad, gouge-like cusps, in place of the slender, 

 pointed cusps of ground snails, and regardless of the form of teeth 

 prevailing in the family stocks whence they were derived. Cases 

 in point are Polymita, Amphidromus, Orthalicus, Papuina, Cochlo- 

 ttyla, Oxychona, etc., etc. Some apparent exceptions are due to the 

 very recent assumption of arboreal habits by certain forms ; the 

 change of teeth lagging behind the change of station, as in the 

 arboreal forms of the genus Cepolis. 



This modification goes hand in hand with the change in shell feat- 

 ures ; arboreal forms always becoming light or bright colored, often 

 having a color-scheme in vivid hues of green, yellow, orange or 

 pink; while the most nearly allied terrestrial species or genera have 

 the shell of dusky or inconspicuous shades of brown. 



In some tree snails the middle cusp only is modified into a broad 

 gouge, the side cusps remaining as rudimentary basal spurs, which 

 become larger on the outer edges of the radula, in accordance with 

 the general law formulated above. An instance is Oxychona. pi. 

 51, figs. 9, 10, (o being the rhachidian tooth). Again, the three 

 cusps are retained and enlarged on all the teeth, as in Polymita, pi. 

 51, figs. 5, 6, 7. (Fig. 7, outermost marginals; compare pi. 57, fig. 

 48, a marginal of Cepolis, the genus most nearly allied). The same 

 has occurred in Papuina, pi. 37, figs. 1, 10. 



As a general rule, groups of greater value than genera cannot be 

 based upon these special modifications of the tricuspid type of teeth. 

 And on account of the fact that similar modes of life produce simi- 

 lar tooth-forms in widely different groups, these peculiarities can 

 have comparatively little weight in fixing the place in the general 

 system or the family affinities of any genus. 



The salivary glands, stomach, liver and intestine have not been 

 observed to offer differences of taxonomic value in the Helices, 

 although I have observed variations in certain genera. An extended 

 series of observations of these organs is necessary. 



KEPRODUCTIVE SYSTEM. 



General considerations: Helicidse, like all pulmonates, are her- 

 maphrodites, the male and female genitalia uniting below in a com- 



