MOLLUSCA. 407 



Family AROIDAB. 



Genus Arca Lamark. 

 Area uniopsis Conrad. 



Plate XXXIV., Figs. 6-8. 



1853. Area uniopsis Con., Jour. Acad. Nat. Sci. Phil., 2nd ser., 



vol. 2, p. 275, pi. 24, fig. 17. 

 1861. Arca uniopsis Gabb, Synop. Moll. Cret. Form.,- p. 154 



(98). 

 1864. Arca uniopsis Meek, Check L,ist Inv. Foss. N. A., Cret. 



and Jur., p. 9. 

 1868. Arca ? uniffpsis Con., Cook's Geol. N. J., p. 725. 

 1886. Cihota uniopsis Whitf., Pal. N. J., vol. i (Monog. U. S. 



G. S., vol. 9), p. 92, pi. II, figs. 32-33. 

 1905. Arca uniopsis ]o\\ns., Proc. Acad. Nat. Sci. Phil. (1905), 



p. 9. ' 



Description. — "Shell rather above a medium size, transversely 

 elongate, and trapezoidal or subrhomboidal in outline, with mod- 

 erately convex valves, which are broadly sulcated in the middle, 

 particularly on the right valve, and only moderately elevated 

 beaks, situated at about the anterior third /of the length, and 

 which in the casts appear to have been scarcely enrolled and 

 moderately distant from each other. Hinge-line not quite as 

 long as the body O'f the shell, in the cast showing characters of 

 a rather low area; anterior end obliquely rounded, receding be- 

 low; basal line broadly sinuate and the posterior end obliquely 

 truncate, longest below the caiter." (Whitfield.) 



The surface marked by radiating costse which are not present 

 upon the internal casts. Muscular scars faintly impressed. 



The dimensions of an internal cast illustrated by Whitfield 

 are: length 50 mm., height 24 mm., thickness 17 mm. 



Remarks. — This species resembles Arca rostellata, and like 

 that species it is rare and usually occurs in the form of internal 

 casts. The casts of this species may be distinguished, however, 

 by their proportionately greater height, the more central position 



