﻿4 



MONOGRAPH OF THE ATLANTlDAEo 



Keel of the shell chalky, as is indeed the whole shell, 

 outer lip of the aperture always more or less fissured; 

 animal with a slender proboscis and a smaller sucker on 

 the fin Atlanta Lesueur. 



Oxygyrus Benson 



Atlanta {p. p.) auctorum. 



1835. Oxygyrus Benson. 



1836. Helicophlegma (p.p.) d'Orbigny. 

 1841. Ladas Cantraine. 



The well-known typical representant of this genus, 

 0. keraudreni (Lesueur), has been classed firstly among the 

 species of Atlanta (so by Lesueur himself, Rang, Cuvier, 

 Deshayes), till Benson established a new genus for it. The 

 diagnosis, given by this author of the type, his 0. in flatus^ 

 which, moreover, has never been figured, is very incomplete 

 and does not show any specific characters, although 

 beyond doubt to be applied to Oxygyrus^ and probably 

 to the common 0. keraudreni. It seems advisable to reject 

 Benson's species. 



I have shown in my monograph (pp. 49 and 50), that 

 Oxygyrus n. sp. Macdonald is the young stage of a species 

 of Souleyet. Leaving aside some doubtful forms (which I 

 shall have occasion to refer to further on), there remain 

 only two species. 



Key to the species. 



Shell cartilaginous for the greater part (in adult state), 

 large (5 — 10 mm.), chalky part of shell without spiral 

 lines; median plate of radula with three spines, of which 

 the middle is the largest one, while the lateral spines 

 are nearly obliterated . . Oxygyrus keraudreni (Lesueur). 



Shell with its cartilaginous part (if present) smaller, small 



1) As to the literature I may be allowed to refer to my former monograph 

 and especially to E. A. Smith, Challenger-Expedition LXXII, 1888. 



Notes from the Leyden Museum, Vol. XXX, 



