i8o 
Memoirs of the Indian Museum. 
[VoL. II, 1909.] 
characteristic of other pediculate fish such as Halicmetus, Dibranchus and Lophius.' 
An essential character may therefore be more liable to change than a trivial one. 
It sometimes happens that two animal forms resemble one another very closely 
as regards their specific characters though differing in some weightier generic charac- 
ter. Such a phenomenon is well known and is usually ascribed to “convergence,” 
— a term of uncertain value. 
It does not, however, necessarily follow that because two forms differ in a funda- 
mental character, while resembling one another in certain specific characters, that 
these latter have been acquired independently in the two cases ; nor does it follow 
that the two forms must have been long separated in their descent. 
The case of Platvtroctegen and Platytroctes , the case of Chaunax pictus and 
C. apus {antea) provide excellent illustrations of this principle. 
' I assume that the “ humeral ’’ spine of Lophius is the same structure under another name, for it 
is based on the opercular bones, and has the same multifid form. 
