130 TEETHING AMONG SELACHIANS. 
to make the collecting of Selachians a principal object of my at- 
tention, and to gathér specimens in greater number than is usu- 
ally the case with animals of these large dimensions. I have 
been richly rewarded for my efforts though it has been at the cost 
of considerable labor. We now possess in the Museum many — 
thousand specimens of Selachians. I do not suppose there is an- _ 
other collection covering so largely the different stages of growth 
of these animals. I have examined these specimens one by one 
and since my return have made a very careful examination of — 
` . one species in several families, in order to have standards of com- 
parison based upon the study of several hundred specimens for 
each family. 
The result of this examination shows that while in their adult 
: condition the Selachians present characters which are very Con- 
a stant among specimens of the same age, such marked changes. 
take place among them during their growth that even genera ha 
been founded on the difference of age. I wish to show first, 
that among the adults we have constancy of character. As an 
example I may take a species of Odontaspis of which I hav 
twenty-five adult specimens. I have selected this genus for s 
cial study on account of its relations to the fossil species of i 
tertiary formation. In Odontaspis the front teeth are placed as 
_ if there were a set of front teeth distinct from the tanine tee! 
and back of them molar teeth of a very different size, all 
—— anche the ‘same ping ghee and ¢ even pe 
esta established, and where the changes are only in the size | 
teeth. There are other genera, on the contrary, in which 
young ma fewer ia and rows of different kinds of teeth 
e is the stefan nieo of development of the teeth 
in which there isse row of teeth standing erect aton 
outer margin of the j jaws, and behind them as many rows © 
wards as s there ar are erect teeth outside. These 
