FOSSILS FROM THE ENVIRONS OF TOKYO. OO 



Shiiiagawa (rare). 



The living species of this genus are cliiefly fourni in arctic 

 regions such as Behring Sea, Ochotsk Sea, Norway, Wellington 

 Channel (North Canada), Kara Sea ; but it also occurs in the 

 United States, Great Britain and the Canaries. In Japanese 

 waters not a single specimen belonging to the genus has ever 

 been collected. 



138. CAKDITA ROTUNDA Tok. 

 PI. III. Figs. 17 a and a'. 



Shell oval, valves nearly equal in length and height, unequi- 

 lateral, exceedingly thick; anterior side rounded, posterior short- 

 ened and somewhat narrowed ; equally rounded on the ventral 

 margin ; surface with from eighteen to twenty radial ribs which 

 are crenulated and separated by fine shallow grooves ; umbones 

 very prominent and directed anteriorly ; margin crenulated ; 

 ligament external ; hinge teeth two, posterior elongated ; palliai 

 line simple ; lunule very small, lanceolate. 



Length 15'"'" 14""" 13'"™ IS'"'" 9.5""" 

 Heio;ht 15'""' 13'"'" 13'"'" 13'"™ 9'""' 

 Breadth 13°"" 10™"' 10™™ 9™™ 7™'" 



In Japan we find two species of Cardita which resemble 

 this fossil one. They are C. ferriiginea A. Adams and C. 

 vesitata Deshayes. But the fossil form is distinguished from 

 C. ferruginea A. Ad. by having a more rounded shape, more 

 prominent and pointed umbones, and less breadth. C. vesitata 

 Deshayes differs from ours in having a very narrow shell (the 

 breadth being only 7.5™™ and the length 17™™) and less promi- 

 nent umbones. C. ventricosa Gld. from Vancouver shows great 



