FROM PLIOCENE CLAY BEDS IN COSTA RICA. 379 



heavy lines of growth, and another has them developed into pretty regular con- 

 centric ribs. 



Length, 3 mm. 



A small, inconspicuous shell, of orthodox shape and appearance, and about 

 which, consequently, little can be said. 



N. MoENENSis, Gabb, n. s., PI. 47, fig. 83. 



Shell minute, TelUniform, nearly equilateral, the beaks central, the posterior 

 cardinal margin slightly arched, the anterior a very little concave, and both sloping 

 at about the same angle. Anterior end produced, obliquely truncated, and causing, 

 by its form, the resemblance to Tellina ; base and posterior end regularly rounded. 

 Surface smooth and unornamented. 



Length, 3| mm. 



This is a remarkable case of imitation of form; and perhaps nobody could con- 

 jecture the true genus, without the actual proof. This, fortunately, I have. The 

 hinge is slender, long, and the regular Nucula teeth, though small, are well marked 

 and numerous. 



PECTEN, Linn. 

 . P. EXASPERATUS, Sbj., Thes. Conch., p. 54, PL 18, figs. 183-186. 

 Rare. Living, Mediterranean. 



JANIRA, Schum. 

 J. MEDIA, Lam. (sp.), A. S. Yert, \. 1, p. 130. 

 Common. Living, West Indies. 



J. L^VIGATA, n. s., PI. 41, fig. 84. 



Shell thin, sub-orbicular, broad ; lower valve convex, equilateral ; surface 

 polished and marked by about twenty scarcely perceptible broad ribs, represented 

 internally by an equal number of pairs of thin threadlike ribs ending a little short 

 of the margin of the shell ; ears nearly equal ; the right ear marked by three or 

 four fine radiating, linear ribs. Upper valve slightly concave, the ribs being indi- 

 cated externally by faint irnpressed lines, crossed by regular, fine lines of growth ; 

 internally this valve has the same character of ribs as the other. 



Length and width, each 1.3 inches. 



SPONDYLUS, Linn. 

 S. Americanus, Lam., A. S. V., v. 1, p. 185. 

 Common. 



