48 MR. A. D. IMMS ON THE DENTICLES [JaU. 17, 



doubtedly placoid scales. I have omitted to give an account of 

 their characters in the various species since they are described in 

 detail for many forms by Steinhard, and I need only add that my 

 own observations are in entii'e accordance with his results. 



3. The facts which 1 have been able to make out lend but little 

 support to the possibility of these structures performing any 

 definite function. There appears to be no relation between the 

 extent of the development and distribution of the denticles and 

 the natiu-e of the food of the various species in which they are 

 found. For instance, in both Galeus canis and Mustelus Icevis 

 the denticles are distributed over an exactly similar area, and 

 there is but a small difi'erence in the form of the individual 

 denticles in the two cases. Nevertheless, Galeus preys on other 

 fish, and has its teeth modified for that purpose, while in Mustelus 

 the teeth are pavement-like, and are used for crushing the shell- 

 fish &c. on which it feeds. 



The spinous portions of the denticles were found in all cases 

 to be directed towards the caudal extremity of the fish, and this 

 renders it possible that the denticles may perhaps serve to 

 roughen the mouth and, by this means, assist in the swallowing 

 of the food. There is also the suggestion make by Steinhard, 

 that they may serve to some extent in grinding up the food, 

 but it is difficult to conceive that they could be of much utility 

 in this direction, for in not a a few cases the denticles are so 

 small as to only produce a barely perceptible roughness to the 

 touch. 



It is possible that the denticles may subserve one or both of 

 these functions, although their value in these respects must be 

 very slight. A more probable view, and one more in accoirlance 

 with their variable disti'ibution and the absence of any obvious 

 correlation between the nature of the food and the presence, 

 a,bsence, or degree of development of the denticles, is that these 

 structures are vestigial organs. It is well known how tenaciously 

 vestigial structures pei'sist, even when they do not subserve any 

 conceivable function, so long as their retention is harmless to 

 the organism. In the case of the denticles, their persistence 

 would not involve any serious tax on nutrition during their 

 development, nor be detrimental in any other way, and under 

 such circumstances, once they had been evolved for any special 

 purpose, the tendency of heredity might be sufficient to secure 

 their retention, even though their primitive physiological value 

 had become lost. The fact that the denticles are relatively late 

 in developing argues strongly in favour of their being vestigial 

 organs. Thus in an Acanthias vulgaris 26 cm. long, although 

 the teeth and dermal denticles were present, oral and pharyngeal 

 denticles had only commenced to develop over a very limited area.. 

 In a Carcharias glaucus 39 cm. long these denticles had not yet 

 attained their full development ; and in a F?'istiurus melano- 

 stomus 14 cm. long no indications of them were to be detected. 



Unfortunately we know nothing concerning the habits of the 



