OF SOUTHERN INDIA. 269 
XXXVI. Family,—RISSOID 42. 
H. and A. Adams, Genera, I, p. 827; Gray, Guide, 1857, p. 96; Deshayes, Anim. s. vert. 
Foss. de Paris, etc., 2nd edit., Vol. II, p. 384; Schwartz von Mohrenstern, Monographie der Familie 
der RissorpEN, Denksch. Akad., Wien, 1861, Vol. XIX, pt. II, p. 71; and 1864, Vol. XXIII, pt. II, 
p. 1; Stimpson, Researches upon the #YDROBIINZ, ete., Smith. Misc. Coll., 1865. 
The Rzssom# appear to be closely related to the Czrrrampx (Sandbergeria 
a. oth.), as well as to the Zirrormip# and Pranaxrp#, and pass on the other hand 
through the fresh-water species into the Virrearipz, and through the amphibious 
forms to the operculated PULMONATA. 
The animals have a produced, occasionally dilated rostrum, which is sometimes 
at its base more or less adnate to the forepart of the foot. In some the eyes are 
sessile at the upper base or near the outer margin of the subulate tentacles, in others 
they are placed on prolonged bulgings, which are for a short or the total distance 
united with the tentacles. The teeth are in seven series, all of them with incurved 
tips; the central being subconic and denticulated like the internal lateral ones; the 
two outer lateral pair are dissimilar, elongated, with converging tips, and also finely 
denticulated. No necklobes are known to occur in any of the species. Foot small, 
roundish, or with a more prolonged basis, at its posterior end sometimes termi- 
nating with short appendices. If the disc of the foot is small, the anterior part 
of the head or rostrum assists them in walking, or rather looping, as is the case 
with many of the litoral.or amphibious species; those species, however, which do 
not live above the low tide mark, and are therefore permanent inhabitants of the 
water, have the foot longer and adapted for sliding. 
Operculum horny, subspiral, with lateral or central nucleus; occasionally 
testaceous with subannular nucleus. 
Shell more or less turreted, small, ovately elongated, rarely depressed ; aperture 
ovate, anteriorly rounded or effuse, with entire margins. 
The Rrssorp# are mostly small shells, which seldom live in deep water, 
but usually on different sea-weeds in the litoral zone. Many of them are also 
inhabitants of brackish and fresh waters, and a small number are only found on 
moist places along the sea-shore, or on banks of rivers, estuaries, &c. Correspond- 
ing to their mode of life seems to be the development of the gills, which undergo 
variations, similar to those noticed in the ZrrrorinipZz. 
With respect to the definition of the family itself, as well as to the number 
and value of the generic denominations, conchologists are by no means agreed; and 
this in spite of the great attention, which these little shells have received within 
the last few years. Species, which are evidently of very similar habits and have 
shells of similar form and structure, are either found distributed in different 
families, or they are by others not acknowledged to be even entitled to generic 
distinction. Such contradictory opinions are daily pronounced in conchological 
publications. 
Sehwartz von Mohrenstern (loc. cit.) has made the family a subject of mono- 
graphic research, and we already owe to his labors monographs on the genera 
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