MONOGRAPH OF THE ATLANTIDAE. 



9 



been already applied, long before, by Costa to another 

 species. The study of Souleyet's specimens has shown me, 

 that they exhibit a series of remarkable features which 

 justify the establishing of a new genus, as a transition 

 between Oxy gyrus and Atlanta. 



Species 1. Frotatlanta souleyeti (Smith). 



1852. Atlanta lamanoni Souleyet, non Escbscholtz. 

 1888. Atlanta souleycli Smith. 



(Plates 1 and 2, figs. 1—6). 



Shell with nearly four whorls, spire rather large, last 

 whorl much enlarging, though not in the proportion as 

 in Atlanta', spire projecting, distinctly visible in side view; 

 at the umbilicus the last half of the penultimate whorl 

 is to be seen, provided with + 10 thin spiral lines, 

 gradually disappearing on the last whorl (fig. 3). 



The cartilaginous keel is very often wanting, as it is easily 

 to be detached from the last whorl, which it embraces 

 almost entirely, but terminates abruptly on the first part. 



The animal resembles Oxy gyrus in many respects (fig. 4). 

 Proboscis very large and often swollen at the anterior 

 part, buccal mass voluminous; tentacles short, situated at 

 the lateral side of the eyes, which have a very broad base, 

 and agree with those of Oxy gyrus. 



Sucker well developed, distinctly separated from the 

 fin by means of a very short stalk ; yet the proportion 

 between sucker and fin is more like that of Atlanta, and 

 so in favour of the last-named organ. 



Radula (fig. 6) strong, as in all Atlantidae. Median 

 plate with three spines, lateral ones directed outwards, 

 and smaller than the median spine. Intermediate tooth 

 with a large median crest, which extends nearly to the 

 tip, resembling somewhat that of Pterotrachea. Lateral 

 teeth both of the same length, slightly curved. 



Operculum (fig. 5) like that of Atlanta, very thin, 

 transparent, cartilaginous. Spiral portion (to which the 



IN'otes from the Leyden Museum, Vol. XXX. 



