54 Mr. 0. S. Tomes. Structure and Development 



Table VI. 



*' Upon the Structure and Development of the Enamel of 

 Elasmobranch Fishes." By CHARLES S. TOMES, M.A., 

 F.R.S. Received February 7, Read February 17, 1898. 



(Abstract.) 



The nature of the hard polished outer layer of the teeth of this 

 group of fishes has been from time to time a subject of discussion, 

 some authors holding that it is enamel, whilst others deny its claim 

 to be so styled. 



The author describes its physical, chemical, and histological 

 peculiarities, calling attention to its hardness, its optical properties, 

 its almost entire solubility in weak acids, and to its tubularity, in all 

 of which respects it resembles unquestionably an enamel. 



But it contains lacunar spaces, and presents a very distinct lami- 

 nation, parallel, or nearly so, with its surface, in which respect it is 

 unlike an enamel. 



Still upon the balance of its characters it has much more in 

 common with enamel than with dentine, from which it is sharply 

 marked off by the entire absence of any collagen basis. 



It is also shown that the tubular structure, which may be regarded 

 as typical in these fish, passes by insensible gradations into a simple 

 tissue differing but little from an ordinary enamel ; this is especially 

 the case where the whole layer is thin, as in the Rays. But the 

 study of its development raises the difficulty afresh. 



