OF SANTO DOMINGO. 



201 



PLANORBELLA. Gabb, ii. gen. 

 Shell minute, vitreous, sinistral, apex sunken as in Plaiiorhis. 



This genxis from its distinctly sinistral character is evidently allied to Limacina, 

 from which its planorbifoi-m mode of growth distinctly separates it. 



p. imitans. Gabb, n. s. 



Shell minute, resembling a broad Plaiiorbis ; apex sunken, whorls two or two and a-half, partially enveloped. 



Except for its sinistral character this pretty little shell might readily be mistaken 

 for a very young specimen of the common Planorhis trivolvus, so nearly does it copy 

 the external form of that species. 



ATLANTA. Lesueur. 



A. r o t u n d a t a. Gabb, n. s. 



Shell minute, dextral ; sjnne minute, sub-globular and obliquely imbedded, nearly enveloped and hidden by the 

 the last whorl ; body volution showing a very minute trace of a keel, barely amounting to more than an obscure an- 

 gulation on the median dorsal line. Umbilicus very shallow and showing but a single volution ; aperture trans- 

 versely elongate ; one end being emarginate by the encroachment of the preceding whorl. 



Its rounded form will serve to distinguish this from all other known species, and 

 seems to necessitate a probable extension of the generic definition, as given in the 

 books, so as to include round-backed, as well as angular or carinated forms. The 

 present species is not absolutely round on the dorsum, birt the keel being rudimentary, 

 shows that such a transition may be expected, and that a generic division on this 

 character alone would not be valid. 



A. c o r d i f o r m i s. Gabb, n. s. 



Shell minute, compressed, sharply angidated on the dorsum and with a thin elevated keel. Apex depressed, 

 not projecting above the body volution. Whorls three ; one and a half being visible in the broad flat umbilicus ; 

 aperture heart-shaijed ; dorsal fissure deep. Sides of the mouth arched forwards in the middle, retreating back- 

 wards with a regular curve to the outer margin. 



ISTot unlike A. Perronii, L. but with a distinctly heart-shaped aperture ; broad on 

 the inner and acute on the outer margins. From A. Kerandrenii, L. it differs in 

 Iseing-a similarly shaped, but much smaller' mouth, and in wanting the lateral angles ; 

 the lines of growth being regularly curved. 



MUREX. Linn. 

 M. d o m i n g e n s i s. Sby. Quart. Jour. Geol. Soc. 18. 



A species approaching Uatistellum in its almost entirely noli-spinous varices. It 

 can be recognized by its comparatively short and crooked canal. 



M. r e c u r V i r o s t r i s. Brod. Proc. Zool. Soc, 1833. 



Living on the West Coast from Mazatlan to Central America. Its straight 

 canal, larger than the preceding, and the few spines on the varices serve to distin- 

 guish it from the preceding. 



A. p. S. — VOL. XV. 2y. 



