1006 MALAYSIAN LAND AND FRESHWATER MOLLUSCA, 
Some of the peculiar genera of this region have extraordinary 
oi’gans which are not seen elsewhere. Thus Opisthoporus is a 
depressed shell furnished with a little open tube behind the mouth. 
Pterocyclos has an almost similar tube formed by a notch in the 
peristome at its junction with the superior whorl, an arrangement 
which is slightly modified in an allied genus named Spiraculum. 
Alycceus has the last whorl swollen, constricted and strongly 
twisted near the mouth. Allthe.se species have peculiar opercu la 
composed of a calcareous spiral series of concentric plates. In 
the family Pupininse there are the strongest modifications of the 
last whorl which becomes twisted and constricted in the most 
erratic manner. In Opisthostoma it is elevated vertically in the 
air like an elephant’s trunk. In Hyhocystis we have a very 
peculiar form of land-shell, of which a full description is given 
at the end of the list. It is an approach to Megalomastoma, and 
may be said to be confined to Burmah and the Malay Peninsula. 
As the limits of the region here dealt with are so little explored, 
no such things as sub-provinces can be made, unless it be in the 
way of considering each island a sub-province in itself. It is 
obvious to any one who considers the size and extent of any of the 
islands, that only a very small portion of them can have been 
well explored for their molluscan fauna. The total number of 
known species or varieties, amounting as it does to scarcely 
400, can only be considered as an instalment of the actual census. 
The large island of Borneo alone might be expected to furnish 
such a number, when we remember how the climate, soil, and 
vegetation of this I’egion favour the development of the molluscan 
fauna. Yet the species of Borneo can scarcely be said to be known 
at all. 
In dealing with the genera and species of the various authors, 
it has already been stated that the specific or generic value in any 
individual case is a matter for which the authors themselves are 
alone responsible. Yet it must be borne in mind that the diffi- 
culty of dealing with some of the larger genera renders sub-division 
of some kind absolutely necessary. Thus in the immense genus 
