182 Zoological Society : — 



Like P. eolina, it differs from all the others I am acquainted with in 

 the upper surface of the whorls being rugose, and only smooth on 

 the lower surface, as is the case with many NaMince, showing, if the 

 smoothness and polish of the surface depend on the extension of the 

 mantle of the animal, that the extension in this kind is confined to 

 the under surface of the shell, as is proved by the examination of 

 the animal itself. 



This behig the case, I am inclined to form this shell and P. eolina 

 into a new genus under the name of Ceres, characterized by the 

 roughness of the upper surface and the non-dilatation of the front 

 edge of the mantle, which is believed to be dilated in all the other 

 species of the true Prosei-pina;. 



It vi'ill be seen that most authors have placed these Mollusca either 

 with Helices or Oligp'ce, and I was much inclined to follow their 

 example, even after a cursory examination of the animal itself. It 

 has much the external ap])earance of the animals of the latter family, 

 having a short, broad, annulated muzzle with a triangular mouth, two 

 subulate lateral tentacles, with the eyes sessile on the outer side of 

 their base ; a moderately short foot, truncated in front, acute and 

 keeled above and behind, without any appearance of beards or any 

 membranous ridge on the sides ; the shell is slightly sunk into a cavity 

 in the front of the upper keeled part of the foot, as if it possessed 

 an operculum ; the edge of the mantle is free from the back of the 

 neck, producing an open nuchal respiratory ca\'ity like Cyclostoma 

 and Oligyra, and other operculated and unisexual land shells. 



When the animal is more closely examined, it is found that there 

 is no operculum ; the concavity on the front part of the foot into 

 which the under surface of the shell fits is furnished with a con- 

 tinuation of the mantle, liaving a raised crumpled edge evidently 

 capable of being expanded over the under surface of the shell, and 

 explaining the polished surface of this part of the shell ; — a structure 

 I have not observed in any other Mollusca. This extension of the 

 mantle might be mistaken for the mantle of the operculum, which, 

 as far as I know, is always quite distinct and separate from the 

 mantle of the shell, but in this animal the fringed edge of the con- 

 cavity is in direct continuity with the true or shell-forming mantle, 

 both at the columnar and the outer external angle of the mouth of 

 the shell. 



The teeth of the lingual membrane are unlike those of Cyclostoma 

 and Helicina, which agree with those of Littorina and other marine 

 Ilostriferous univalves. The teeth resemble those of the typical 

 Raphidoglossa, as in the families Neritinidoi, Turhonidce, Trochidce, 

 Eoliolidce, &c. All the Mollusca hitherto known belonging to these 

 families are aquatic, and all but the genera Neritina and Navicellus 

 are truly marine. They all have well-developed gills, and the 

 greater part have a more or less developed lateral membrane on each 

 side of the bod}', furnished with three or more beards on its lower 

 surface, and almost all have the eyes placed on a more or less distinct 

 peduncle at the outer side of the base of the tentacles, all characters 



