174 PALAEONTOLOGY OF CALIFORNIA. 



CARDITA, Brug. 



C. IIORNII, G. 



PI. 24, Fig. 157. 



(C. planicosta, Con. (not Lam.) Pacific R. R. Report, vol. 5, p. 321, pi. 2, fig. 6.) 



Shell large, thick, convex, subquadrate, oblique ; beaks pro- 

 minent, anterior, subterminal ; cardinal margin broadly arcbed, 

 sloping slightly, and uniting with the posterior end with a regu- 

 lar curve ; base broadly rounded, most prominent in the middle, 

 from which point it runs upwards rapidly towards the anterior 

 end, which is broadly and regularly curved; posterior end ob- 

 liquely subtruncated, angular below. Surface marked by twenty- 

 two broad rounded ribs, a little the smallest posterior to the um- 

 bonal angle ; these ribs are somewhat flattened above, especially 

 towards the base, have acute interspaces, and are crossed by 

 numerous coarse, irregular lines of growth. Hinge very thick, 

 robust, and resembling that of C. planicosta of the Eocene. 



Figure, natural size. 



Localities: Abundant near Fort Tejon; also found at Martinez, and near 

 Clayton. 



I dedicate this magnificent species to my friend, Dr. Horn, IT. S. A., in recog- 

 nition of the valuable assistance he has rendered me, in collecting Cretaceous 

 fossils in the vicinity of Fort Tejon. 



It is not so surprising as might appear at first glance, that Mr. Conrad should 

 have referred this shell to Lamarck's species. The form figured is probably an 

 extremely oblique one, although all of the adult specimens show more or less of 

 an approach to this shape. The young shells, of which I have examined a large 

 number, approach much more nearly to C. planicosta in outline. Add to this the 

 fact that whenever the shell is weathered so as to lose the outside layer, the ribs 

 become quite flat, angular on the sides, and with broad, flat interspaces. Wher- 

 ever the surface is preserved intact, however, the rotundity of the ribs, and the 

 narrow, acute interspaces, show a character entirely incompatible with the other 

 form. A further resemblance is in the hinge, which, as well as my memory 

 serves me, is nearly, if not quite, undistinguishable from that of C. planicosta. 



