ZOOLOGY. MOLLUSCA. 197 



cutting points ; the side cusps short, attaining about the mid- 

 dle of the basal plate, and directed outward; the lateral teeth 

 are similar, but lack inner cusps; they are about eight in 

 number, and are followed by about four transition teeth; the 

 marginals number about fifty on either side, their cusps 

 become more slender toward the outer edge, and the basal 

 plates shorter. A central tooth, with five adjacent lateral teeth, 

 and a group of transition teeth, with a true marginal tooth, are 

 shown in the figure. 



Helix albella of Chemnitz (not of Linnaeus) and H. ochro- 

 leuca of Pfeiffer (not Ferussac) are, I believe, synonymous with 

 this species. The former is placed in Eurycratera in Pfeiffer's 

 Nomenclator, and the latter has been compared to Pachystyla 

 rufozonata, a form somewhat similar in characters of the shell, 

 but belonging, of course, to a distinct group. 



Pcecilozonites Nelsoni, Bland. (PI. 16, figs, j, K, L.) 



A fossil form, differing from Bermudensis in its much greater 

 size, the greater number of whorls, more convex base, coarser 

 striation, impressed sutures, and especially in the peculiarly 

 prominent dome-shaped upper whorls. These are, indeed, so 

 closely coiled as to resemble a specimen of P. circumfirmatus. 

 The coloration, imperfectly shown in several specimens before 

 me, is that of Bermudensis; and whilst its affinities are with 

 the latter species, I regard it as a divergent branch, rather than 

 as an ancestor of that form. 



As has been observed in other cases of species approaching 

 extinction, and probably subject to some decided and unfavor- 

 able changes of environment (in this case, perhaps, due to the 

 comparatively recent subsidence and partial submergence of 

 the island*), the shell exhibits great mutations and distortions 

 of form ; sometimes the spire is elevated conical, sometimes 

 much depressed ; frequently the planes of the upper and lower 

 volutions are not parallel, and the spire is consequently canted 



* See Challenger Report, Narrative, vol. i, p. 138. 



