JUKES-BROWNE : CEETACEOTJS AND EOCENE VENEEID^. 



175 



living animal. The depth of the sinus, however, depends on the length 

 of the siphons, and in most cases the length of the siphons depends on 

 the depth at which the animal buries itself below the surface of the 

 sea-floor. The sinus therefore only represents a functional character, 

 and not one of any great phylogenetic importance. 



As a matter of fact I have been impressed with the great variations 

 in the depth and form of the sinus exhibited by some fossil species of 

 Veneridse, which must, from their other characters, be grouped in the 

 same genus. In some cases this variation extends from a slight 

 sinuation to the presence of a deep and ascending sinus. It seems 

 reasonable to conclude that some of these species had accustomed 

 themselves to live at a much less depth below the surface of the sand 

 than others, and consequently that their siphons had degenerated into 

 short non-retractile tubes. Whatever the reason, the fact remains, 

 and consequently I do not think that a shell should be placed in 

 a separate genus merely because it has no pallial sinus, and when it 

 agrees with other species in all other essential respects. 



On the whole it appears to me that those characters of the animal 

 which impress themselves on the interior of the shell are not of generic 

 value, though they are useful in the establishment of subgenera and 

 sections. These characters are the sinuation of the pallial line and 

 the scars of the adductor and pedal (or protractor) muscles. In the 

 discrimination of genera more satisfactory results can be obtained by 

 giving preference to certain characters of the shell itself ; these are 

 (1) the hinge-plate and its teeth, (2) the features of the lunule and 

 escutcheon, (3) the smoothness or crenulation of the valve-margins. 



In accordance with these considerations I would tabulate the 

 various genera and subdivisions which have been described in the 

 preceding pages as follows : — 



Genera. 



Sections. 



Subgenera. 





Dosiuiopsis. 









[Bositiia.^ 







Sinodia. 







I Calpitaria. 



Pit aria. 





Callista. 



1 Macrocallista. 

 ( Callistina. 



Aphrodina. 





Tlvelina. 



( Meretrissa. 

 \ Bollfusia. 













[Callocardia.l 



Atopodonta. 







[Sunetta.^ 





Meroena. 





Circe. 



Circenita. 



Gouldia. 





Ptychomya, 









Clementia. 





1 Flaventia. 

 \ Psathura. 





Cyprimeria. 





Cyclorisma. 





Chione. 





Textivenus. 





Mercimonia. 









Baroda. 





Venerella. 





Tapes. 





Veneritapes (?). 





