336 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



A genus is not a definite creation, but a result of evolution, and 

 the establishment of genera is therefore largely a matter of convenience. 

 Thus, when we are dealing with a large group of shells it is con- 

 venient to divide it into several genera, if these can be usefully 

 defined, wliile, if this very group had not developed so many species, 

 it might have been more convenient to make one genus with several 

 sub-genera. It is not necessary that a genus should be an absolutely 

 isolated group ; a genus may be isolated from other recent genera by 

 the extinction of links, though some of these links may occur in fossil 

 faunas ; on the other hand, iti some cases links between two generic 

 groups may have survived to the present time, and for convenience 

 may have to be included in one of the two genera. 



The Cytherea or Callista group is a large one, and consequently it 

 seems convenient to divide it into a limited number of genera, 

 provided that these can be defined so that they are easily distin- 

 guishable from one another. Both Komer and Dall have essayed 

 to do this, and Homer's three divisions seem to me much more 

 natural and convenient than Dall's. Thus I see no reason for 

 separating what Dr. Dall calls '■Agriopoma'' ivom. Pilar ia, the shells 

 having substantially the same dentition; while the typical Dione 

 group agrees much more closely with Amiantis than with Pitaria. 

 Finally, Aphrodina is essentially a Callista, as I have previously 

 shown, and has so little in common with the Venus puerpura group 

 that I am sure few will follow Dr. Dall's lead in that direction. 



An independent examination of a large number of species 

 belonging to these groups led me to the conclusion that tliey 

 might conveniently be arranged in three sets which could well be 

 regarded as genera. Subsequent reference to Homer's lists of species 

 showed me that my grouping was substantially the same as his, 

 though I can certainly indicate the critical differences between them 

 more briefly and clearly than lie did. 



These three genera have been widely known by the names of 

 Callista, Dione, and Caryatis, but here we are brought up against 

 that thorny and almost insuperable obstacle of nomenclature. 

 Callista cannot be used as if it liad been properlj- established by Poli, 

 because he applied it solely to tlie animal, or rather to the animals of 

 several Linntean genera, and did not intend it to be used as a name 

 for any shells belonging to the Linnaean genus Venus. Again, if ttie 

 strict rule of priority is enforced Callista cannot be derived from 

 Morch or Adams because Gray unfortunately published Leach's 

 use of it in 1852, though so used it becomes merely a synonym of 

 Clausina (Brown). 



The rule of priority, however, is breaking down from the shear 

 weight of the absurdities and inconveniences with which it is 

 burdened, and of these Callista is a striking instance. If discarded 

 it would have to be replaced by the name Macrocallista, which 

 presupposes the existence of a Callista and was actually proposed 

 by Meek for a mere section of Callista. The larger and typical 

 group has then to be included under the name Chionella, which 

 was proposed by Cossmanu so late as 1887 for a small group of 



