GASTEROPODA. 2]\ 



Synonym, Acme and Acmaea, Hartmann.* 



Shell ininuto, slender, nearly imperforato ; peristome sligMly 

 thickened, margins sub-parallel, joined by a tkin callus ; oper- 

 culum hyaline. 



Distribution, 7 species. Britain, Germany, Prance; Yanicoro 

 (on leaves). A. fusca is found in low, marshy situations, at the 

 roots of grass ; it occurs fossil in the Newer Pliocene of Essex. 

 (J. Brown.) 



Geomelania, Pfeiffer. 



Type, G. Jamaicensis, Pfeiffer. 



Etymology, Ge, the ground {i.e. terrestrial). 



Shell imperforate, turreted ; aperture entire, effused ; peri- 

 stome simple, expanded; margins joined, basal ]3roduced into a 

 tongue-shaped process ; operculum oval, pellucid, whorls few, 

 rapidly enlarging. 



Distrihution, 21 species. Jamaica. 



Oeder III. — Opistho-branchiata. 



Shell rudimentary or wanting. Branchice arborescent or 

 fasciculated, not contained in a special cavity, but more or less 

 completely exposed on the back and sides, towards the rear 

 [opisthen) of the body. Sexes united. (M. Edwards.) 



The molluscs of this order may be termed sea-slugs, since 

 the shell, when it exists, is usually small and thin, and wholly 

 or partially concealed by the animal. "When alarmed or 

 removed from theii' native element, they retract their gills and 

 tentacles, and present such a questionable shape that the inex- 

 perienced naturalist will be likely enough to return them, with 

 the refuse of the dredge, into the sea. Their internal structure 

 presents many points of interest ; in some the gizzard is armed 

 with horny spines, or large shelly plates ; in others the stomach 

 is extremely complicated, its ramifications and those of the 

 liver being prolonged into the papillte, which are said to be 

 branches of the respiratory organ. The tongue is always armed, 

 but the number and arrangement of the lingual teeth is ex- 

 ceedingly variable, even in the same family ; usually the dental 

 membrame is broad and short, with many similar teeth in each 

 row. 



The lingual dentition is extremely varied in the Bullidce. In 



* All given in the same year, 1S21, the name Acmaea having been employed by 

 Escbscholtz for a genua of limpets ; Acicula has been retained by Pfeiffer and Gray 

 for this land-shell. 



