604 
FORMS 
anomalous case of a very limited area inhabited by several peculiar genera, each of which 
is represented by a large number of species. 
During a late visit to the island, I enjoyed the opportunity of examining the rich local 
collection of Major Skinner, a collection which, including a large number of specimens 
of several of the peculiar species, afforded excellent data for determining how far the 
numerous forms described by recent authors were really distinct species, or merely 
varieties linked together by intermediate forms. Major Skinner informed me that, in the 
case of two of the endemic genera {Aulopoma and Tanalia), he had found it impossible to 
discover any constant association of characters by which one species could be satisfactorily 
separated from another, and that many of the described species of Achatina and certain 
Helices presented difficulties of the same kind, increasing as he obtained collections from 
a greater number of localities. Having myself, while collecting in Southern India, noticed 
the great variability of certain species, and that in many cases, owing to the describers not 
being provided with intermediate forms, varieties of the same shell had been described 
under different names (e. g. Helix fallaciosa, Perussac, H. ruginosa, Perussac, and H. crassi- 
costata, Bens. ; H. Tranquebarica, Pabricius, H. semirugata, Beck, and H. vitellma, Pfr.), 
I addressed myself during my stay in Colombo mainly to the examination of such puzzling 
species, with a view to trace out the connexion of varieties by the comparison of a large 
number of specimens, or to fix the limits of variation so far as the collection permitted. 
How necessary such revision has become in the case of many of our Indian shells any one 
who has collected over an extensive area, and endeavoured to trace out the geographic 
range of species, will readily acknowledge. In a subsequent paper I purpose contributing 
such corrections of Indian species as my own collections have enabled me to establish ; 
in the meantime I give the results of my examination of the genus Tanalia, the numerous 
described forms of which are all peculiar to Ceylon. In another paper I shall submit a 
similar notice of the Aulopomas and certain Cyclophori *, Helices, and Achatinas. The 
Melanias and Ampullarias equally require revision ; but these must be considered in 
connexion with Indian species, most of them having a very extensive range. 
Genus Paltjdomits. 
Subgenus Tanalia, Gray, 
I have little to add to the description of this subgenus, given by Mr. Layard, except to 
notice the variability of the form of the operculum. This appendage is sometimes broadly 
triangular, the angles being more or less rounded, sometimes compressly elliptical ; and 
the nucleus, usually about the centre of the dextral edge, or a little inferior, is, in one 
species [or variety ?] , T. violacea, Layard, at the dextral corner of the base, and sometimes 
projecting below the basal edge. A monstrous form of the operculum (possibly that upon 
which Mr. Layard founded the subgenus Ganga) is not infrequent : in this, the nucleus 
Monograph 
faction of seeing that he has therein made many of those corrections in the described species, of the necessity of which 
I had already convinced myself. 
