MONOGRAPH OF THE ATLANTI DAE. 



3 



May this paper contribute something to our knowledge 

 of the group, and facilitate in any way the task of future 

 investigators — a task which I know by experience to be 

 by no means an easy one. 



The genera of the Atlantici a e. 



Two genera are generally admitted, Oxygyrus and Atlanta, 

 which are distinguished by a whole series of characters. 

 After the study of Souleyet's types I have thought it 

 necessary to add a third genus, Protatlanta, the type of 

 which is represented by Souleyet's Atlanta lamanoni, which 

 name has been altered by Smith, for reasons of priority 

 in nomenclature, in Atlanta souleyeti. This remarkable new 

 genus forms in many respects a transition between Atlanta 

 and Oxygyrus, but it cannot be classed among either of 

 these genera. 



Key to the genera. 



1. Shell nautiloid, all whorls in the same plain, horny 

 to a greater or lesser extent, according to age; keel mem- 

 branous, nearly as broad as the last whorl ; operculum 

 triangular; animal with a very bulky proboscis and a large 

 sucker on the fin Oxygyrus Benson. 



Shell right-handed ; apical whorls forming a little spire 

 at one side of the flat shell, an umbilicus existing at the op- 

 posite side; operculum rounded, oval, with a spiral portion . . 2 



2. Keel of the shell cartilaginous, encircling nearly the 

 whole last whorl (but often wanting as it is most easily 

 to be removed) and extending to the outer lip of the 

 aperture; shell quite 1 ) cartilaginous (?) ; animal very much 

 resembling that of Oxygyrus, with a mighty proboscis 

 and a large sucker Protatlanta mihi. 



1) The specimens of Souleyet, which are very well preserved, show no 

 trace of chalky matter in their shells. On the other hand, Smith (p. 44) has 

 stated, that the shell is „of the same vitreous character" as in Atlanta. This 

 question may therefore remain unsettled. 



Notes from the Leyden Museum, Vol. XXX. 



