JTJKES-BEOWNE : ON GALLISTA, AMIANTIS, AND PITARIA. 337 



Eocene fossils. This is surely a reductio ad alsurdum ! For the 

 prevention of such cases as this it seems to me that a relaxation of 

 the rule is required, and that authority should be given for deriving 

 the name from the first subsequent author who used it for the same 

 group to which the current type belongs. This would enable us 

 to pass over Leach and to derive the name Callista either from 

 Morch, whose first species was C. erycina, or from H. & A. Adams, 

 ■whose typical example was C. chione. I shall therefore retain the 

 name Callista in the hope that the International Zoological Congress 

 will eventually adopt this course. 



The name Dione, however, cannot stand because it was preoccupied 

 by Hubner in 1816 (Lepidoptera), nor can it be replaced by Hystero- 

 conclia as Dr. Dall proposed, fur this name is not Fischer's as he 

 supposed. Fischer in iiis Maniiel (1887) used Dione, and only 

 mentioned Uysteroconcha of " Lang, 1722 " in brackets as a synonym, 

 the date showing this name to be pre-Linnaean. I shall demonstrate 

 in the sequel that the Dione group is more closely connected with the 

 Amiantis of Carpenter than witli the Pitaria group. Consequently, 

 I agree with Rouier in regarding the two former as members of one 

 genus, the name of which will be Amiantin. 



With regard to the name CaryatU, this was also preocciipied by 

 Hubner in 1816; moreover, Homer had himself previously proposed 

 the name Pitar for the group, and tliis, as amended by Dr. Dall and 

 converted into Pitaria, shouhl certainh' be accepted. 



The criteria on which I mainly rely for the establishment of these 

 three genera, Callista, Amiantis^ and Pitaria, are (1) the existence 

 in the two latter of a channel leading from the pit between the 

 anterior laterals of the right valve below the anterior cardinal into 

 the first interdental socket, (2) the position and shape of the left 

 posterior cardinal, (3) the form and direction of the pallial sinus. 

 Neither the characters of the external surface nor the bridge- 

 connexion of the anterior and posterior cardinals in the right valve 

 are to be depended upon, though of course they are useful points in 

 diagnosis. So also is the existence of a definite escutcheon in Amiantis. 



Relying on the characters of the dentition only, the three generic 

 groups maj' be distinguished as follows : — 



In Callista there is no connecting channel ; the left posterior 

 cardinal is short, high in the middle, and confluent with the nymph. 



In Amiantis there is a channel under the right anterior cardinal ; 

 the left posterior cardinal is long, highest at the end, and confluent 

 with the nymph (as in Callista). 



In Pitaria there is a channel as in Amiantis, but the left posterior 

 cardinal is generally more or less separate and slightly cui'ved, so as 

 to extend across the hinge-plate to its inner margin. 



There are, however, some exceptions to these rules or generaliza- 

 tions. Thus, Callista aurantiaca. Sow., has the fosse and channel of 

 an Amiantis, though in all other respects it is a Callista. It may, 

 therefore, be regarded as to some extent a link between the two 

 genera, but I do not propose to give it a sectional name, as that would 

 be magnifying the importance of a single structural character. 



