. FROM PLIOCENE CLAY BEDS IN COSTA RICA. 365 



stronger than the others, A rather sharp angle runs on each side from the apex, 

 bordering the dorsal surface, ending abruptly against the outer posterior angle of 

 the aperture. Borders of the mouth roughened by irregular lines of growth. 



Length 3 mm. 



There being but one specimen of this shell in the collection, and these forms 

 being very variable, often depending for their shape on the character of the sur- 

 face to which they are attached, I feel reluctant in giving a specific description ; 

 not doubting but that the ilext specimen may be of a very different form. Never- 

 theless it is important that all the organic remains in these rocks be made known, 

 even if imperfectly, so as to enable us to arrive at the more important geological 

 results. 



ADEORBIS, S. Wood. 

 A. SIMPLEX, Gabb, n. s., PI. 46, fig. 61. 



Shell minute, discoidal, flattened, whorls four, nearly flat on top, bending 

 down slightly to the suture ; base a little more convex, periphery regularly and 

 narrowly rounded; umbilicus broad; surface smooth. Mouth a little wider than 

 high ; outer lip sinuous. 



Figure magnified ten diameters. 



A perfectly plain, unornamented little shell, unlike, in this respect, all the 

 others of similar form or size with which it is associated. 



A. INFEACARINATA, Gabb, n. s., PI. 46, fig. 62. 



Shell minute, discoidal ; spire slightly elevated ; whorls three to three and a 

 half, convex on top; suture nearly obsolete; periphery bearing a strong rib, 

 below which, on the under surface of the whorl, are three more ; the innermost 

 forming a faint margin to the umbilicus, and being placed on the middle of the 

 base. Umbilicus broad and deep, showing all the whorls. Mouth oblique. 



Smaller than the preceding shell, this cannot be confounded with it, even in 

 the top view, where they resemble each other in being smooth. In that, the 

 suture is very distinct ; in this it is faintly marked. 



The figure is magnified twelve diameters. 



I have tried to identify this shell with Guppy's Vitrinella marginata. His de- 

 scription gives no idea of the size, and his figure must therefore be taken as the 

 only clue we have. My species is li mm. in diameter, while his figure is over 6. 

 They agree in having four ribs, invisible from above ; but from Guppy's figures, 

 these ribs seem to be confined to the periphery, and do not reach the under sur- 

 face. But, on the whole, the figure quoted is so very imperfect that very little 

 can be learned from it. 



