a “ef another character come in, These remain for a time 
132 _ ‘TEETHING AMONG SELACHIANS. 
ological formations. I have been fortunate enough to secure a 4 
large number of specimens of ‘the Cestracion living along the 
coasts of California, Peru and the Galapagos Islands. I have 
those of Australia also in various stages of growth, so that I could | 
ascertain the mode of dentition of the genus by a comparison of 
different species. In the adult, as is well known, the front teeth | 
are pointed, while the lateral teeth are grinders, and there are 
* grinders with flat surfaces and grinders of different forms, in the 
middle of the jawand behind. What is particularly characteristic i 
of these fishes is that the teeth rest upon the surface of the jaw 
forming flat expanses for chewing, and that many teeth are at work 
at the same time; also that the inner part only of the rows is i 
progress of formation, while a great many rows act at the same 
oct this condition, the genus Cestracion has been ded 
and it is generally understood that what distinguishes it is the 
portunity to examine the younger ones, I found that there were 
none of those peculiar teeth in the back part of the jaw. Y€ 
that swim about in search of prey exhibit only the conical teet 
the anterior part of the jaw and have none of the teeth with 
surfaces at the hinder part of the jaw. What is still more § 
is that these front teeth, corresponding to the front teeth of 
adult, have not single points as in the adult but two lateral 
The teeth which are behind are gradually reduced to * 
prongs, and finally only one prong with a little hook on the 
and in very old specimens even these little prongs at the side 
wanting; so that you have a succession of different teeth 
_ sulting from the gradual change in the teeth of the same § 
The first teeth of the young have this complicated cha 
_ which is maintained through successive droppings before the 
until a third type of teeth is brought in. As these chan; 
in the front we find that row after row is added bebind, 
ee number of rows covering the surface-of the = 
i 
We see in 1 this ndiferent arrangement from the other 
in which the total number of teeth in the jaw is early 
_ and remains the same for life, while here the number 
incr ipoe il: the TOWS s behind have a ao 
