220 PSEUDOTROCHUS. 



Shell imperf 'orate, long-ovate, with conic spire and obtuse 

 apex ; rather thin, smoothish or plicatulate, with more or 

 less obvious fine spiral incised stride; covered with a thin 

 cuticle, which is often evanescent. Whorls 6-7, the first ones 

 smooth, with no sculpture except faint growth-lines; the last 

 whorl rounded or angular at the periphery; the suture nar- 

 rowly bordered below by a band, which is almost always 

 crenulate or beaded. Aperture oblique, ovate, the outer lip 

 simple, unexpanded, often thickened within; columella sub- 

 vertical, more or less distinctly truncate at the base. 



Type, P. alabaster (Rang). Distribution: West Africa, 

 from Liberia to Gabun, especially on the Gold Coast ; Prince 's 

 Island. 



Perideris, the name universally current for this group since 

 1856, the date of Shuttleworth 's luminous monograph, was 

 preoccupied by Brandt in 1835, for a Holothurian. An- 

 other generic term must, therefore, be found for the mol- 

 luscan genus. 



Chersina was first used by Humphrey, in the anonymous 

 invoice of the Calonne collection, for some 18 species of 

 Achatinoid snails, beginning with species of Liguus, and 

 including Achatina, Ampliidromus, and probably Oxystyla, 

 Strophocheilus and other genera; though only the first three 

 genera are represented by species identifiable by the quota- 

 tion of Linnean names. Beck, in 1837, adopted "Chersina 

 Humph." for a group including Perideris alabaster Rang 

 and the species of Liguus; and Albers, 1850, restricts it to 

 the species of Liguus. While I do not regard the Calonne 

 sale catalogue as authority for names either generic or 

 specific, yet the absence of species of Perideris in that work, 

 as well as the possibility that some authors may consider it 

 quotable in nomenclature, render it inadvisable to use Cher- 

 sina Beck for the species alabaster and its allies. It seems 

 better to leave Chersina Beck in the synonymy of Liguus. 



Pseudotrochus was one of the pre-Linnean names exhumed 

 by H. and A. Adams, who used it for species of "Perideris'' 

 and Liguus. Their first species, and one of the two figured 

 as examples, is alabaster Rang; and by elimination of the 



