34 PLEIOCENE FOSSILS. 



Fam. AKCAOIILE. 



ARCA.—Linn. 

 ARCA HIANS.— {New Species.) 



Plate XFV., Figs. 4 and 5. 



A. testa oblonga, compressa, radiatim costata ; latere buccali, brevi, rotundato ; latere anali, 

 elongate-, sub-compresso, expanse-, sub-angulato oblique truncato ; latere palliali hiante ; den- 

 tibus cardinalibus minimis; area ligamenti, excavata, sulcata; umbonibus approximates. 



Description. Shell oblong, compressed, radiately costate, buccal side short, rounded; 

 anal side elongated, dilated, somewhat angulated, truncate ; pallial margin gaping ; cardinal 

 teeth small ; ligament area deep, channeled ; umbones approaching. 



The ribs are marked by impressed lines, which become obsolete towards the buccal and 

 anal margins. The whole shell is crossed by incremental lines, which are raised and some- 

 what squamose towards the pallial margin, and along the hinge line on the anal side. The 

 cardinal teeth are small and irregular, and become, with age, at the extremity of the hinge, 

 mere points. The buccal muscular impression is deep. In old individuals the pallial mar- 

 gin is undulating and toothed, and at all ages gaping towards the buccal side. 



Resembles A. propatula, Con., from which it is distinguished by its greater relative 

 length, and by being more angular at the extremities. In A. propatula the ratio of 

 breadth to length is 6-10. This belongs to the genus Byssoarca, of Swainson. The open- 

 ing for the byssus is quite large. 



Plate XIV., Fig. 4. Exterior of upper valve mid hinge and teeth of lower valve. 

 " 5. Pallial margin to show gaping. 



Locality. Sumter District. 



Museum, College of Charleston. 



