10 



MONOGRAPH OF THE 



ATLANTIDAE. 



musculous part of the tail is inserted) with a short spiral Hue 

 and a few concentric ones; distal half provided with some 

 parallel striae. 



As may be inferred from the foregoing description, 

 Protatlanta with the only species P. souleyeti (Smith) , 

 forms in many respects a remarkable transition between 

 Oxygyrus and Atlanta. To repeat it shortly : the cartilaginous 

 consistence of the shell and the keel, the form of the last, 

 the proboscis and the tentacles, are like that of Oxygyrus ; 

 the projecting spire, the proportion between sucker 

 and fin, and the spiral operculum, are all features, which 

 resemble Atlanta. 



Diameter of the shell 2 mm. or less. 



The species seems to live only in the Atlantic, 

 from which it is recorded both by Souleyet and Smith 

 (Challenger-Expedition). 



As I had only five type-specimens of Souleyet at my 

 disposal, I have not ventured to search for the mucous 

 glands of the mouth, which occur in Atlanta (recorded by 

 me recently, p. 51, PI. VII, fig. 8). 



Atlanta Lesueur. 



1817. Atlanta Lesueur. 

 1825. Steira Eschscholtz. 

 1868. Atalanta Knocker. 



Shell and keel chalky, not flexible as in the foregoing 

 genera, outer lip of the aperture fissured. Animal with a 

 slender proboscis and long tentacles. Operculum with a 

 spiral portion. 



For further particulars 1 may refer to the diagnosis I 

 have given in my monograph (pp. 50 and 51); I shall 

 only add, that the keel is made up of two plates (as in 

 Carinaria), leaving, at least on the last whorl, a small 

 space between them. This keel becomes gradually lower 

 towards the aperture, and disappears entirely at the fissure 

 in the outer lip. 



3>Totes from the Leyden Museum, "Vol. XXX. 



