﻿MONOGRAPH OF THE ATLANTIDAE. 



7 



becomes very low and incouspicuous on the first half, 

 though reaching the inner lip, and so embracing the whole 

 circumference of the shell. The separation of the cartilaginous 

 and the chalky part of the shell is not quite clear, but 

 it seems as if the membranous matter gradually covers 

 the chalk and absorbs it. Sculpture is altogether absent. 



Though there is a gap in my material between the shells 

 of 3 mm. and of 5 mm., the idea that 0. rangi represents 

 a young stage of 0. keraudreni is very suggesting, also, 

 because 0. ra7}gi has been caught by Mr. Buitendijk in 

 the Mediterranean, which has always been regarded as the 

 typical habitat of 0. keraudreni. 



But a few facts must be born in mind. Firstly, the 

 Seller opImia-stsLge of 0, keraudreni, as it is figured by 

 Oberwimmer, exhibits spiral lines, but these do not undulate, 

 as is decidedly the case in 0. rangi. Such simple lines I 

 have not observed in any specimen. And secondly, the 

 radulae are indeed very different. I have pointed to it 

 formerly (monograph, PL VII, figs. 3 and 5), and can only 

 confirm, after repeated investigations, that in 0. keraudreni 

 the median plate carries three spines, the middle one of which 

 is large, while the lateral ones are inconspicuous, whereas in 

 0. rangi these spines are of nearly the same length. 



To the genus Oxy gyrus, and probably to 0. keraudretii, 

 another species, » Atlanta violacea'' Gould, from the tropical 

 Atlantic (copied by me, PI. I, figs. 42 and 43) is likely to 

 be referred; at least the shell is said to be nautiloid, with 

 the last whorl »not rapidly enlarging, and unusually 

 distended" ; the keel is very high at the aperture (which, 

 however, is elliptical, not rounded) ; and finally, the side-view, 

 showing an umbilicus at both sides of the shell, the 

 violaceous colour, especially on the spire, and the diameter 

 (9,5 mm.) strongly suggest the idea that the species must 

 be classed in Oxygyrus. 



The same is true perhaps for another species of the 

 same author, ^ Atlanta tessellata\ also from the tropical 



Notes from the Leyden Museum, Vol. XXX. 



