418 PROCEEDINGS OF THE ACADEMY OF [1896.. 



There remain only the Bulimoid forms ; these being mostly fossil 

 have received little attention in the usual works of reference. The 

 well known Bulimus miris-vulpina o^J)i\\\syn (sp.) was erected into 

 a genus by Beck as early as 1837 under the name of PachyotusJ 

 With it Beck associated a number of species of the type oi Bulimus 

 bilabiatus and melanostomus, which arrangement was followed by 

 PfeifFer and most subsequent writers. There are distinct points of 

 resemblance, but these are probably dynamic rather than ontogen- 

 etic. To the writer the relations of Pachyotus are directly with a 

 certain number of its associates of St. Helena. 



The Bulimiform Helicacea of St. Helena may be divided into two 

 groups*: — one (Achatiuoid) typified by the Pachyotus auris-vulpina 

 and characterized by a closed or nearly closed umbilicus and a cork 

 screw twisted axis, the other (Bulimuloid) by a straight axis and 

 more or less open umbilicus. The last group comprises Bnlimulus 

 Blofieldi and Seleanus of Forbes and B. lielena Quoy and Gaimard. 

 The Pachyotus group^ comprises all the other species of the island. 



' The type was selected from among Beck's species by Gray in 1847. In 

 1848 Fischer de Waldheim named it CJdlonopsis. 



* According to Mr. H. A. Pilsbry, whose opinion on the subject is entitled 

 to the greatest weight, the two principal groups are probably referable to the 

 AchatinidiE {Pac]iyotus ^xiA Cleostyla) an\i the BuliiiiulidcE [Fachnodiis) . The 

 former would be nearest to Ferideris, and the latter to Pachnodus as typified 

 bj' P. vehdhiHs. As the so-called To7>iigerus oi'^t. Helena is probably a mod- 

 ified Pupa [Cai/ipolamics Pilsbry) analogous to Boysidia and Hypelostoma, ii 

 would seem that the affinities of the St. Helena fauna are West African, Ori- 

 ental or Oceanic, rather than South American, in spite of the presence in 

 South America of the Achatinoid " B^ coronatus and " B." HanUyi Pfr. 



^ The teeth of the radula of P. melanioides are in nearly straight transverse 

 rows, and the rhachidian teeth are narrow, with a single small cusp, as in 

 typical Achatinidu-. The laterals are bicuspid. On the marginal teeth the outer 

 cusp splits, forming two or three denticles on the inner, four or mo^e on the 

 uter teeth. The formula is 14.12.1.12.14. The arcuate jaw (distorted in my 

 preparation) is very closely and finely striated, as in Liniicolaria. 



XV 2S- 26 



t^^ rj: ■ -^1^ W"^' — 



Teeth of Pachyotus melanioides. Jaw of P. melanioides. 



The main character of the dentition different from other genera of Acha- 

 tinidcr is the multiple splitting of the outer cusp on the marginals, as in Cion- 

 ella^ Pupidcr, Vallonia, etc. — H. A. P. 



