PSEUDOTROCHUS. 221 



species of lAguus, which had long before been segregated by 

 Montfort, only "Perideris" remains. It seems proper, un- 

 der the circumstances, to revive Pseudotrochus, in a re- 

 stricted sense, for alabaster and its allies. 



A young specimen of Perideris has been reported from 

 Ujiji (P. Z. S., 1880, p. 352), collected by Hore. This is so 

 far out of the known range of the genus that there seems a 

 possibility of misidentification owing to the ambiguous char- 

 acters of a young shell. 



Shuttle worth, in his Notitia (1856), gave an excellent 

 account of the species then known. His results were adopted 

 by Pfeiffer in subsequent volumes of the Monographia Heli- 

 ceorum. The only recent monograph is that of Kobelt, 

 1893-4, who describes 19 species. The number is herein in- 

 creased to 24. 



1. P. ALABASTER (Rang). PL 16, figs. 70, 71, 73, 74. 



Shell ovate-conic, solid, gray-white, with an opaque-white 

 sutural border, and on the las't whorl one or several olive- 

 yellow or brown bands of cuticle at or below the periphery, 

 and a narrower one below the suture. Surface smoothish, 

 marked with faint growth-lines and minute spiral incised 

 lines. Spire straightly conic, the apex obtuse. Whorls about 

 61/2, nearly fiat, joined by an almost even suture, the margin 

 below it not crenulate. Last whorl varying from rounded 

 to angular at the periphery. Aperture ovate, oblique, white 

 inside ; peristome thickened within; columella vertical, white, 

 truncate at the base. 



Length 36-40, diam. 18 mm. 



Length 32.5, diam. 16 mm. 



Length 40, diam. 22 mm. (Rang). 



West Africa: Prince's Island (Rang, Folin, Welwitsch, 

 Vignon) ; all over the southern part of the island, on the 

 leaves of trees and bushes, between 15 and 20 ft. up (Dohrn). 



Helix alabaster RANG, Annales des Sciences Naturelles, 

 xxiv, p. 20, pi. 1, f. 2, 2a, Paris, 1831. Achatina alabaster 

 DESH., in Lam., An. s. Vert., viii, p. 312; in Fer., Hist., 

 p. 150, pi. 124, f. 7, 8. REEVE, Conch. Syst, ii, pi. 178, 



