Mr. W. H. Benson on the Shell and Animal of Hybocystis. 91 



neous operculum of Megalomastoma, but distinguished by the 

 thickness of the circular peristome from the typical forms. 



I am indebted to Capt. Haughton, late magistrate of Moulmein, 

 for two specimens of M. gravidum, each with the operculum in 

 situ. Its nature and construction at once announced that the 

 separation of the form from Megalomastoma was absolutely 

 necessary. The thick calcareous shield is many-whorled exter- 

 nally, and peculiarly convoluted on the inner surface. The 

 generic term " Hybocystis '' has reference to the form of the 

 typical shell. 



A comparison with the thin, similarly convoluted operculum 

 of Otopoma Blennus, B., from the same locality, induced a sus- 

 picion that the latter shell was merely the young of Hybocystis 

 gravida, notwithstanding the abrupt termination of the last 

 whorl of that part in O. Blennus ; and finding, on forcibly with- 

 drawing an operculum, that the foot, head, tentacula, and eyes 

 adhering to it were quite fresh and moist, and evidently in a 

 living state, and the operculum of one of the specimens of Hyb. 

 gravida opposing even greater resistance to the attempts made 

 to separate it, I took measures for the re-animation of the ani- 

 mal, as well as of other specimens of O. Blennus ; and, in spite 

 of the apparent emptiness of the whorls, with the exception of a 

 small portion near the aperture, I succeeded in making the 

 animals of both forms move about freely, when their absolute 

 identity in colour and conformation placed the fact of their 

 being the adult and young of the same species beyond a doubt. 

 The form of the now obsolete species, O. Blennus, coincides with 

 that of the spire of Hybocystis gravida before it assumes its 

 lengthened and distorted phase. 



In treating H. gravida as the type of a new genus, I have been 

 guided by the following considerations, in addition to the struc- 

 ture and substance of the operculum. The character assigned to 

 Hainesia, Pfr., viz. the oval mouth, angulate above (as recorded in 

 the * Mai. Blatter' for 1856, and partly copied in the 'Monogra- 

 phia Pneum.' Supp. 1858), would at once exclude from that sec- 

 tion or genus the Moulmein shell, even if any of the three species 

 included in it, in consequence of possessing that sole feature in 

 common, should be found to be provided with a calcareous oper- 

 culum. I doubt whether the two West Indian forms in the 

 section will present opercula formed on the same type as that of 

 the Siamese shell with which Dr. PfeifFer has associated them ; 

 but as the diagnosis of the section is quite inapplicable to the 

 Burmese species, I have no hesitation in conferring on the latter 

 a distinct generic appellation. If eventually M. Myersi should 

 exhibit an operculum similar to that of Hybocystis, it will neces- 

 sarily have to drop its connexion with Hainesia, the name of 



