PLEIOCENE FOSSILS. 



NOTE ON THE GENUS ARCA. 



D'Obbigny has, we think properly, re-united to Aeca the genera Byssoarca and Cucul- 

 L2ea, as only modifications of that genus. As for the character dependant on the teeth, it 

 varies so widely in Abca as to render it often difficult, if not impossible to fix the limits 

 between this genus and Cucull^ea. 



The gaping species were referred to Byssoabca, under the impression that such alone had 

 a byssus, but this is not the case ; we have ourselves dredged up in Tampa Bay groups of 

 Abca transversa, a closed species, attached to dead shells by a byssus. The gaping 

 species, according to D'Obbigny, lose the byssus with age. 



The genus Abca is first found in the Silurian rocks, and has its greatest developement in 

 the existing seas. 



