250 



Cuban shell was named Gundalachia Ancyliformis, and is the 

 ; of interesting articles by Bourguignat in the Revue et Maga- 

 Zoologie, 1855, p. 23, and 1862, p. 13. A second species G. 

 HJalmansonii, from Honduras, has since been described by Dr. 

 PfeifFer. (Zeitsch. flir Malak., 1852, pp. 179-84 ; and Malak. 

 Blatter, V. 196.) M. Bourguignat, in the latter of the two ar- 

 ticles above quoted, has described two additional species from 

 Cuba, G. Poeyi and G. aderosia, and in the Proceedings of the 

 California Academy of Natural Sciences, for April of the present 

 year, Mr. Howell has described and figured a fifth species, G. 

 Ccdifornica, from the waters of the Feather River, of which a 

 cut is here oiFered (fig. 1), copied from Mr. Rowell's figure. 



In the spring of the present year, I have discovered a sixth 

 species in the vicinity of Washington, D. C. The genus is thus, 

 as far as known, peculiar to North America and confined to 

 the tropical and warm-temperate regions. 



I have taken the opportunity afforded by the occurrence of 

 the last-mentioned species, to study the lingual dentition of the 

 animal and the mode of growth of the shell in this curious ge- 

 nus. The species may be called 



GUNDLACHIA MeEKIANA. 



Fig 2. The full-grown shell, in general 



form, is ovate. It is much broader 

 than in G. Ancyliformis, and has a 

 less ovate aperture than in G. Cali- 

 fornica, as may be seen by compar- 

 ison of the figures. 

 The shell consists of two distinct parts, and from above looks very 

 much like a small and thick, black Ancylus, sticking obliquely and to 

 the right upon the posterior end of the back of a larger thin and 

 whitish one. These two parts we will call, for convenience, respective- 

 ly the smaller shell and the larger shell. The two parts nearly 

 resemble each other in outline, each being oblong, roundedly truncate 

 before, and narrowed and somewhat obliquely truncated behind, the 

 right posterior angle being prominent. 



The dorsal part, or smaller shell, as before stated, is black opaque, 

 and comparatively thick. It is about one-third as long as the larger 

 shell, and has the usual form of a young Ancylus, the very obtuse apex 

 being at the posterior third of its length and inclined to the right. An- 

 teriorly it is continuous with the dorsum of the larger shell, but poste- 

 riorly it projects freely over and beyond the njiargin of that shell at its 

 posterior dexter angle, to a distance equalling rather less than a fourth 



