178 Records of the Indian Museum. [Vor. Vile 
to cite them as instances of convergence produced by isolation, 
the ancestors of different species isolated by some means from one 
another having, in the absence of enemies and the presence of a 
liberal supply of food, tended to revert in general structure to their 
common but long extinct ancestor, while retaining certain unimpor- 
Fic, 5.—-Branchial skeieton of Emyda cranosa, xX 2. 
tant distinctive features. Theoretically it would be difficult in 
that case to regard such forms as local races of one species, but in 
practice this seems at present to be the only possible course to 
adopt, if we are to pay any attention to geographical considerations 
in distinguishing between subspecies and varieties. (See Annan- 
dale, Fauna Brit. Ind.—Freshwater Sponges, etc., p. 18, IQII.) 
