GASTEROPODA. oll 
Synonym, Acme and Acmaea, Hartmann.* 
Shell minute, slender, nearly imperforate; peristoume slightly 
thickened, margins sub-parallel, joined by a thin callus; oper- 
culum hyaline. 
Distribution, 7 species. Britain, Germany, France; Vanicoro 
(on leayes). 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 produced into a 
tongue-shaped process; operculum oyal, pellucid, whorls few, 
rapidly enlarging. 
Distribution, 21 species. Jamaica. 
ORDER III.—OPISTHO-BRANCHIATA. 
Shell rudimentary or wanting. Branchie 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. Hdwards.) 
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 
remoyed from their 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 
Sa 
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 papillee, 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 Bullide. In 
* All given in the same year, 1821, the name Acmaea having been employed by 
Eschscholtz for a genus of limpets; Acicu/a has been retained by Pfeiffer and Gray 
for this land-shell. 
