61 



LuciNA. (8p. undt.) 

 Plate IX., fit;-. 12.* 



Fig. 6. 



Compare Luviiia xubiniJata, Hall and Meek. " Memoirs of the American Academy 

 of Arts and Sciences." Cambridge ; 1856. Page 384, Plate I., figs. 6, a, b. Also, 

 Meek's " Report on the Invertebrate Cretaceous and Tertiary Fossils of the Upper 

 Missouri Coimtry." Washington : 1876. Page 133, Plate XVII., figs. 2, a, b, c, d, e,/; 

 especially 2,/. 



Shell compressed, thin, .sub-cireular, but mure or less augiiUir ; broad 

 in front, narrow behind ; length rather greater than the height. The 

 beaks, which arc placed a little on the posterior side of the middle, are 

 not very small ; they project distiijctly above the liighest level of the 

 hinge line, and are directed forwards. There is no clearly defined 

 lunule, and the escutcheon is merely a linear lanceolate groove, with 

 obtuse margins, for the reception of the ligament. The sujierior border 

 is broadly triangular; the anterior side is Avide and somewhat squai'ed, 

 but the upper angle is slightl}- in advance of the lower, and the latter is 

 the most rounded off of the two. The basal margin is gibbous in front, 

 but abruptly contracted behind; the posterior side narrows rapidly both 

 above and below ; its extremity being squarely truncate. 



The surface is so much worn that the only markings visible are a few 

 faint concentric stria? of growth ; the characters of the interior of the 

 valves are unknown; the test is extremely thin. 



Greatest length of the only specimen, nine lines ; height, from the 

 beaks to the base, about eight lines; maximum thickness, four lines. 



The differences between this little shell and some examples of 

 L. subundata are very slight. The posterior half of the large individual 

 of the latter species, figured by Mr. Meek in the Eeport quoted above, 

 (at Plate XVII., fig. 2, /,) almost exactl}' corresponds with that of 

 the present fossil, but the shape of the anterior side of the two shells is 



* As this flsfure does not convey a verj- accurate iJea of the shape of the shell, a more correct 

 outline is given in the woodcut. 



