176 BULLETIN OF THE 



nacreous deposit increases with age, the thick flat labium and elevated 

 pillar tooth always indicating maturity. 



The mature reflected lip of Partula always exhibits numerous inter- 

 rupted microscopic lines, running in the direction of the peritreme. In 

 all thin-shelled species the surface is more thickly crowded by the 

 waved spiral striae, common to all the species, than in the heavy and 

 thick-shelled varieties : in the latter, they are not so conspicuous, partly 

 owing to the decussation of the more coarse oblique lines of growth. 



The shells of the same species of Partula often vary in color. Al- 

 binism is often present ; but the species vary through all the shades of 

 dark bay to pale or reddish chestnut, greenish yellow, rufous, hyaline, 

 and white. 



The colors, which are deposited by glands on the margin of the man- 

 tle, are not found in the embryo, but after extrusion and exposure to 

 light and heat the colors appear; and owing to the latter influences, 

 the arboreal species are more beautifully marked than the terrestrial. 

 The colors of the shell are ai-ranged in streaks, rays, or bands, the 

 latter varying in number and width, while the former follow the direc- 

 tion of the whorls, becoming wider as the shell increases. Like Acha- 

 tinella, some species possess a white sutural line beneath the whorls 

 of the spire, and the uniform dark purple or rose tint of the apical 

 whorl is a marked feature in the coloration of many species. In the 

 embryonic shell the fine spiral striae of the epidermis at the apex (when 

 viewed by a glass) are seen to consist of spiral rows of fovea?, or separate 

 depressions in the epidermis, which occupy the first one and a half or 

 two whorls (the usual number they possess when extruded from the 

 oviduct). After birth the fovea? are discontinued ; but we see in their 

 stead the fine spiral waved stria? common to all the species, the em- 

 bryonic fovea? always remaining at the apex of the shell, constituting a 

 generic feature common to all Partula? with which I am acquainted. In 

 Partula, as in some species of Helix, Bulimus, Achat inella, and many vivip- 

 arous fresh-water genera, as Palndina and Lanistes, we meet with what 

 are termed sinistral or reversed individuals. We can only conjecture as to 

 the cause of this departure from the more usual conformation ; but it may 

 be owing to a reversal of the vital forces acting during the segmentation 

 of the yolk of the egg in the early stages of the formation of the embryo. 

 The eggs of the common garden slug (which are almost transparent, and 

 afford good material for observation) a short time alter deposition exhibit 

 the germinal vesicle (which lies in the midst of the yolk) rising to the 

 upper part, where a, distinct rotation may he seen ; after which it under- 



