1916.] J. HorNeLL: Indian Varieties and Races of Turbinella. 121 
to deep water undisturbed by surface disturbances. If this be so, then the invasion 
of the Palk Bay region by acuta, by way of Pamban Pass and Adam’s Bridge, will not 
prove successful; the two varieties will continue to crystallize their characteristics 
and will end, as they were undoubtedly doing up to the time the land barrier broke 
down, in becoming distinct species. 
The phylogenetic relationship between the various varieties as indicated by mor- 
phological considerations supported by distributional and geological evidence may be 
represented as follows :— 
var. obtusa var. globosa var. acuta var. fusus 
var. comorinensis 
Hypothetical 
common ancestor. 
The strongly marked periostracal layer in acuta may possibly be correlated to 
either or both of two reasons, the greater need to protect the shell in the Gulf of 
Mannar from the erosive effects of the coarse gritty sand characteristics of typical 
chank beds in that region or from the destructive burrowing of the tunnelling sponge 
Cliona. The thinness of the periostracum in the Coromandel race of T. pirum obtusa 
may be correlated to a decrease in the activity of these noxious factors in the Coro- 
mandel Sea—a doubtful hypothesis— or it may be related to the accessory assistance 
rendered against such factors by the very frequent occurrence of a crusting hydroid (?) 
on the shells from this region ; this adventitious coat certainly aids in the protection 
of the shell from corroding influences whenever it occurs. All these are however mere 
guesses in the present state of our kuowledge of the subject. 
