162 University of California Publications in Zoology [Vol. 13 



in a young heterodont, twelve to fourteen weeks of age, first appear 

 as pigmented areas which upon being touched with a dissecting needle 

 are easily destroyed. In fact, after the tip of the scale has perforated 

 the integument the scale remains so plastic as to be readily bent in 

 any direction. Such scales arising on the margin of the fin, on the 

 supraorbital crest, or on the ventral side of the body where the ex- 

 posure is great, have their size and form greatly modified by the 

 environment. 



This may be further illustrated by the ventrals. We have noted 

 above (page 159) the fact that the ventral scales of the young in the 

 middle line are sharp-pointed and that the spines extend abruptly 

 downward. It is clear from the habits of the young that these scales, 

 although plastic, would be little exposed since the body weight in the 

 standing position falls largely on the marginals of the paired fins, and 

 that as a result of this protection the spines of the ventrals would 

 be free to grow abruptly downward. As the body becomes heavier 

 the scales would be more subjected to pressure, whereupon they would 

 become flattened into the spade-shaped type. 



(&) Stomodeal denticles in relation to function. — Considerable in- 

 terest attaches itself to the question whether or not the stomodeal 

 denticles show any considerable functional modifications, for the ques- 

 tion has often been raised whether or not the stomodeal denticles are 

 themselves functional (see Steinhard, 1903; Imms, 1905). As to indi- 

 cations of function in the denticles as judged by functional modifi- 

 cation, it may be said that in an immature specimen most of the scales 

 both on the floor and roof of the mouth have the spines sharp-pointed 

 and hence do not show signs of wear. In the anterior part, however, 

 near the symphysis (pi. 8, fig. 1) the denticles are somewhat flattened. 

 In an adult heterodont, while there is more evidence of modification, 

 especially in the anterior part of the roof, this, it seems to me, is hardly 

 suflEicient to indicate that the scales have any considerable function. 



I am of the opinion that the stomodeal denticles have their real 

 explanation in the history through which they have passed rather than 

 in the function which they may now subserve. The mouth, formed 

 as a pitting in from the outside, has carried in the integument as a 

 lining. This lining gives rise to these structures as it would have 

 done had the skin remained external in position. While scales such 

 as the supraorbitals hypertrophy, because of exposure, scales located 

 inside the buccal cavity, as is indicated by their irregularit3^ tend to 

 atrophy. 



