1896.] NATURAL SCIENCES OF PHILADELPHIA. 419^ 



Those who are not accustomed to recognize the flexibility of organ- 

 isms nor to discriminate ancestral from dynamic characters will^ 

 perhaps, be astonished at any arrangement which includes in one 

 group species apparently so dissimilar as auris-vulpina and melani- 

 oides, but I think a little unprejudiced study of the specimens, in 

 connection with B. subplieatus, will convince any one of the likeli- 

 hood of their genetic relationship. 



To treat the simplest and smallest group first, we may take the B, 

 helena and its allies. This species was placed in the section Ncesio- 

 tus by Pfeiffer in 1856 (Mai. Blatt., II, p. 161); and it is quite similar 

 in several respects to some of the Galapagos species, but is probably 

 derived from another shoot of the genus Bulimulus. The nucleus 

 in this species, in B. Blofieldi and Seleanus, is swollen and almost 

 smooth. It has no axial dimple and the surface seems not to have 

 had any coarse sculpture. The species show the microscopic irreg- 

 ularity of the incremental lines, the undue thickening of the shell 

 and the broken lines of spirally disposed granulations which indicate 

 the influence of an arid or alkaline habitat. Full grown specimens 

 generally show the irregularities of the aperture characteristic of 

 individuals which have been forced into long continued hibernation 

 before the mantle had discharged all its surplus calcareous salts, or 

 had, by reason of long continued aridity, to caulk the vicinity of the 

 aperture with shelly matter in order not to be absolutely desiccated 

 by evaporation. These characters are precisely those we find im- 

 posed upon the Galapagos, Lower California and other arid region 

 species. 



The Achatinoid group though possessing many dynamic charac- 

 ters in. common is probably derived from two sources. B, exulahis 

 Benson and B. siibtruncatus Smith have an imperforated twisted 

 axis, a plump small nucleus followed by a few small and then sev- 

 eral rapidly enlarging whorls, a flaring aperture angulated below 

 and with a keel or angle on the edge of the pillar. They show les& 

 than any of the others the effects of aridity and have rather thin 

 shells. They originally had translucent or brownish shells with a 

 pale tracery of opaque white or yellowish. The aperture is regular 

 and there is but little callous deposit. For this section the name 

 Cleostyla may be used. Its resemblance to Pseudachatina seems to 

 be slight and superficial. 



The second group, Pachyotus of Beck, with C. auris-vulpina asr 

 type, comprises also B. melanioides, B. subplieatus and probably B^ 



