364 
On Fresh Water Testacea. 
[Dec. 
the ditch is entirely dependant on rain-water for its supply, it is liable, even during 
the rainy season, to drought, and during the remaining eight months of the year it 
is as perfectly parched as the neighbouring soil ; yet in a very few days after it has 
become replenished with water, the little Paludiiua may be seen clinging to the 
stems of the grass which then grows in it. 
Perhaps the thick calcareous operculum of the Paludina , which fits closely to 
the shell, shuts it in from all communication with the external air, and enables it to 
resist evaporation, its own fluids supplying it with sufficient moisture to sustain life 
in a torpid state, during the prevalence of drought. In Ibis manner a Bulimus* 
(a land suail), by means of a viscous fluid which it secretes, fastens itself to plane 
surfaces ; between which and its shell, it interposes a thin membraneous layer, 
composed of the hardened viscus, and may be found adhering to rails or the trunks 
of trees ; exposed to the fury of the hot winds, the animal only dissolving its barrier, 
and venturing forth to feed upon the herbage when the rains have fully set in. 
According to my second supposition, the animal may be torpid in the earth, beneath 
the surface of the place where the pool stood, or in a state of extreme aridity, and 
may revive on the approach of moisture ; as land shells, which have been for years 
in ablutions in a similar state, and, which is more to the purpose, naked polypi, 
which live in water, have been known to do. The retention of life under analogous 
circumstances by fish and insects, in the states of lawa and pupa, submitted to 
severe cold and congealed to a solid mass of ice, is not more extraordinary. 
Under either of these circumstances a difficulty still remains, in what manner 
did the original progenitor of these molluscs, reach the spot ? We must trace its 
transportation to a period, the memory of which has been lost in the lapse of ages, 
and which perhaps was anterior to the first peopling of the country, before the cave 
excavated its present deep bed, and when it was spread in the form of a lake over 
the spot in question, and a large tract of the neighbouring country. Of the former 
existence of this lake, abundant evidence is afforded by the appearance of the surface 
of the country, and the vestiges of inundation, particularly at the top of the elevation 
on the side of which the ditch is cut, where, at the depth of 3 or 1 feet, occur 
numbers of specimens of shells of a species of Unio, coinciding exactly with one of 
the species which now live in the cavef. At a somewhat lower level SJelumce*, 
Flanorbes% and other shells similarly circumstanced occur imbedded in the soil 
in profusion. In some instances these shells are invested with a concretion of 
sand and calc-tuff which sometimes wears a crystalline appearance. 
Should it be found on enquiry that the Paludina conceals itself in the soil , (which 
discovery is only likely to be made by chance, in consequence of the smallness of 
the shell,) a close examination and subsequent immersion in water will test the truth 
of the first two suppositions. 
It is possible, however, that the ova, which in general have been found to retain 
vitality under exposure to much greater degrees of heat and cold than perfect ani- 
mals, and which may he in this species a gelatinous substance, might be dried up 
to a form capable of being transported by storms, by which bodies much heavier 
are removed; but in this case the growth of the perfect animal from the ova must be 
very rapid. 
The shells which I have hitherto mentioned belong to Lamarck’s class of Mollusca, 
or the univalves of authors; hut at Hamirpfir I have observed Conchifera (bivalves) 
in a shallow tank near the J umna, to which neither the W'aters of that river, nor of the 
Betwa have risen for a half century ; which is 12 or 14 feet above the highest rise of 
the rivers inordinary seasons, and which has no communication with any other 
water. There are small and fragile, almost membraneous, species of Anodonta , 
which I have never met with in rivers, but have found them in other tanks and ponds, 
to which no stream can have had access since the period when the diluvial waters 
* Bulimus. Ovate-conical, right lip sharp, whorls flattened, white, stercorate, 
with a single red-brown hand at the middle of the lower whorl. Length » inch. 
Species B. Gl. Sci. p. 264. 
Icon. pi. viii.y. 4. The length is taken from the largest specimen discovered. 
f Unio. B. Gl. Sci. p. 205. Icon. pi. vii./. 1. 
It is found imbedded indifferently in loam and beds of calc-tuff gravel. In one 
specimen which I possessed, from the latter deposit, part of the shell has been 
changed, throughout its thickness, into calcareous spar, which retains the original 
form of the shell, even to the rugie on the exterior ; the rest of the shell appears 
only to have been deprived of its gluten. 
J Melania. B. Gl. Sci. p. 264. Icon. pi. vii. f 8. and 9. 
U Planorbis. B. Gl. Sci. p. 264. Icon. pi. viii./. 14. 
