remarks on conchology and malachology. 235 
** 
There is a speeies of Planorbis or shell allied to Planorbis found 
here in pools of fresh water, being the only speeies of Singapore 
shell that is found solely in fresh Water; the outer whorl is little 
more than a quarter of an inch in diameter, aperture of the shell 
more diagonal than is usual in Planorbis so that when the animal 
moves on a plain surface the convex side of the shell is always 
uppermost whereas the animal of Planorbis is described as carry¬ 
ing its shell erect or with the diameter perpendicular; colour of the 
shell pale amber, no operculum, animal nearly black, mouth ver¬ 
tically cleft, no perceptible neck, (in the animal of Planorbis, the 
Reck is said to be elongated) eyes at the base of two blunt ten- 
tacula in which also it differs from Planorbis which is commonly 
described and figured as having two subulate tentacula: the animal 
possesses in a considerable degree, the power of gliding through the 
water, apparently in search of food, with its shell entirely submerged 
and its smooth foot in close apposition with the surface of the water, 
locomotion being effected, by causing the flat part of the shell to act 
on the water in the manner of a fin, the head of the animal being 
at the same time directed forward so as to regulate its movements; 
the animal does not oceupy so much as half the shell, and the re¬ 
maining space frequently contains air, which the inhabitant has the 
power of expelling at pleasure. 
The Genus u Natica"’ of which there are several elegant species 
in Singapore, is known from the a Ncrita” or a hoof shell” by being 
umbilicated, more rounded in form, and the interior not toothed, the 
shell has been also described as having no epidermis, to this rule 
however, there are marked exceptions, two of the species native here, 
having a strongly adherent epidermis. 
In Swainson’s Malachology there is a species figured as an extra¬ 
ordinary animal, much larger than the shell it is supposed to in¬ 
habit, one of the species found here presents the same appearance in 
a remarkable degree; and the phenomenon is caused in the fol¬ 
lowing manner; the interior of the foot of the animal, is of a 
loose cellular texture, which it has the power of distending with 
water so as to be more than three times the hulk of the shell, 
but on the approach of danger it can instantly reject the water, 
assume its natural size, and retreat into its shell closing after it the 
operculum which being of stony hardness - ; secures it from the attack 
