4 



MONOGRAPH OF THE ATLA NTIDAE. 



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 cartilagiuous 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. 



r»Jotes from the Leyden Museum, Vol. XXX, 



