420 PROCEEDINGS OF THE ACADEMY OF [1896. 



Turtoni, though the latter is less certain and may possibly belong to 

 a third section. The typical Pachyotus has a dimpled nucleus, 

 though it is not keeled like that of N<xsiotus and the Lower Califor- 

 nian and Peruvian Bulbmdi. In its sculpture the transverse pre- 

 dominates over the spiral. The surface of the shell is everywhere 

 transversely wa-inkled and toward the suture is more or less gathered 

 into short rounded plaits, stronger in the younger whorls. The 

 colors are dark, more or less translucent tesselated with paler opaque 

 markings or streaks. The axis is minutely tubular and twisted, 

 especially as the last whorl is being finished off, where at maturity 

 a plait is more or less distinctly developed. 



The aperture has a simple, somewhat expanded, more or less thick- 

 ened edge, which, in old specimens which have hibernated, may 

 show heavy deposits of callus, which is always angulated or obscurely 

 channelled at the base of the pillai'. Specimens which have sur- 

 vived hibernation have the usual irregularities about the margin. 

 A careful inspection reveals no reasons for supposing that P. auris- 

 vulpina might not have been the descendent of a form like P. 

 melanioides. I have seen no completely adult specimens of the lat- 

 ter or of B. Turtoni, but see no reason to suppose that the lip would 

 not, under suitable conditions, be thickened in them as it is in 

 P. subpHcatus. Perhaps at present P. vielamoides inhabits a region 

 where it does not now suffer from aridity, which would account for 

 the difference in the deposit about the mouth. It is well known 

 that great fluctuations have taken place in the rainfall on the island 

 due to variations in the woods and forests, their destruction and 

 partial restoration. However this maybe, the living species of the 

 group have but little callous deposit about the mouth ; P. suhplicatus 

 which evidently from the freshness of its colors, cannot have been 

 long extinct, has a greater amount, and P. auTis-vulpina in addition 

 to the marginal thickening shows a parietal tubercle of callus often 

 of large size, and the irregularities of form, size, and margin of the 

 aperture are such as to indicate clearly degeneration leading to ex- 

 tinction by increasing aridity of its habitat. 



Curiously enough, according to Mr. Smith, only one Helix, an 

 introduction from other regions, has been found in Ascension Island ; 

 the other known terrestrial mollusk is Limax ascensio)iis Quoy, 

 which may well be an introduction also. The explanation of this 

 difference between St. Helena and Ascension lies in the greater 

 aridity of the latter. Though thorough search might reveal some 



