MONOGRAPH OF THE ATLANTIDAE. 13 



by the keel; shell usually colourless, without spiral lines, 

 sometimes attaining a size Of nearly 10 mm. 



Atlanta peroni- growp. 

 Shell with a short conical spire, always projecting beyond 

 the last whorl ; keel generally not reaching the inner lip 

 of the aperture; sculpture often present in the form of 

 spiral lines on the apical whorls; shell small, with a 

 maximum size of 3 — 4 mm., usually smaller .... 2 



2. Spire straight (as regards the plain in which the last 

 whorl is rolled up) or very faintly reflexed backwards, 

 conical, with rather obtuse apex, generally coloured with 

 brown or yellow tints, which vanish almost entirely on 

 the last whorl Atlanta in flata- group. 



Spire reflexed, either backwards or forwards, in various 

 ways of distinction 3 



3. Shell horny-coloured, spire always darker, slender; 

 keel extending nearly to the outer lip, fissure therefore 

 scarcely developed Atlanta turriculata-grouy. 



Shell colourless; spire very distinctly reflexed, outer lip 

 deeply fissured Atlanta inclinata-grouip. 



Group of Atlanta peroni. 



Five species are to be distinguished here : 



1. A. peroni Lesueur. 



2. „ steindachneri Oberwimmer. 



3. „ affinis Tesck. 



4. „ gaudichaudi Souleyet. 



5. „ lesueuri „ 



They are all characterized by their flat shells, which 

 are nearly planorboid. The first three are (in adult state 

 at least) remarkable by the keel, which separates the last 

 whorl from the penultimate one, penetrating more or less 

 between the whorls. In A, gaudichaudi and A. lesueuri the 

 spire is extremely small, in comparison with the last whorl. 

 For reasons mentioned above, I shall not try to give a 

 key to the species of this group, which are recognizable 



Notes from the Leyden Museum, Vol. XXX. 



