THE SCALES IN SOME TELEOSTEAN FISH. 65 
by Huxley’s own criterion of exogenous or endogenous mode 
of growth, I think the entirely enderonic origin of the whole 
teleostean scale must be admitted. It would seem that a 
difficulty had presented itself to Huxley, since he regards the 
nature of the deeper layers of the scale as uncertain; for he 
writes (11, p. 386) that it is “an open question whether the 
deep layers of all scales are produced by a continuation of 
the process” (i.e. of a calcification of the ecderon), “or 
whether in some cases a deep truly enderonic structure may 
be added to this superficial ecderonic constituent to con- 
stitute the perfect scale. A process of the latter kind would, 
at any rate, find its parallel in the eventful union of the teeth 
of many fishes with their jaws, and in that of the plates of 
the Chelonia with the vertebral elements.” 
The view which I have advanced seems in no way to 
invalidate the conclusion as to the homology between teeth 
and scales. In the former there are both dermal and epider- 
mal constituents as represented by the dentine and enamel 
cap ; the same is seen in the scales of Selachians and Lepido- 
steus. In the latter the dermal portion grows upwards to 
reach the epidermis and there receives its enamel addition. 
In the teeth of the higher vertebrates the process has become 
more specialised, and the enamel germ grows inwards to 
meet the uprising dermal papilla. In the T'eleosteans con- 
ditions are similar to those obtaining in the Selachians, but 
owing to the smallness of the spines and their failure to reach 
the level of the epidermis, no enamel tip is consequently 
formed. 
There remains yet one further point to which I would refer, 
namely, that raised by the recent work of Hoffbauer and 
Stuart Thomson (17). These writers believe that there is an 
annual growth of the scale in rings, which therefore furnish 
an index of the age of the fish. Thomson likens this annual 
growth to the annual rings in the stem of a Dicotyledon. 
Neither the idea nor the simile are by any means new; both 
were, as has been shown, originally suggested by Leeuwen- 
hoek in 1685. 
