i TEETHING AMONG BELACHIANS. .. 188 
, character from those in front. The front teeth in these rows con- 
stantly drop and give place to others. This implies a functional 
differentiation which is marvellous. Mark that every tooth that 
replaces another has a different character from the previous one. 
We have here opportunities for variation, for changes, for trans- 
mutation, to an extent which has. not been noticed in any other 
family of animals as far as I know. 
Let me here say that from single.specimens of Cestracions, ob- 
tained in different parts of the world, have been indicated three 
supposed genera based on the conditions of the teeth at different 
periods of age. 
To show that this should not be EP as an unquestion- 
able result, let me say that I have examined the young of the 
three supposed genefa. They are all provided with keeled molar 
teeth, while the adults have the flat grinders supposed to be 
characteristic of the Cestracion type alone. I am therefore satis- 
fied that it is worth while to collect largely and preserve a num- 
ber of specimens, even if they be sharks and skates and occupy a 
great deal of room, in order to learn their history, which has 
shown of just what importance has been the identification of teeth 
among fossils. Thus sharks drop their teeth and scatter them 
along. the bed of the ocean in great numbers, probably ten or 
twenty times as many as they have at one time while living, so 
‘that it is not to be wondered at that we so frequently find in col- 
lections of fossils loose teeth of sharks, and that we so rarely find 
_ the jaws of sharks with teeth i in their places. 
Of course in those species in which the teeth are isolated and 
do not support one another, we should hardly ever expect to 
find them fossil in positio; while those which are pressed upon 
one another may be found in the fossil state, and that occurs 
again and again, and among the fossil fishes there are a num- 
ber of sharks in which gil witir w arranged in rows are’ 
~ represented. ; 
There is a third mode of toting ša different from the other 
_ two as these two are different from one another. It occurs in the 
aes of Myliobatid among the skates. In Myliobatis, as gen- 
_ erally known in the mature-condition, we have.a middle row of 
_ broad and short teeth; along the margin are A-number of rows of 
smaller teeth, and it is known that in front thése teeth drop and - 
from behind. oo ea 
oe b 
