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TEETHING AMONG SELACHIANS. 131 
, functionally used in catching the prey there correspond three, four, 
five, six or seven immature teeth placed one behind the other in the 
- fold of the gum, the youngest occupying the innermost portion. 
These teeth come into play one after another as the front teeth 
drop off. This is the mode of teething among Selachians with 
which all zoologists are familiar, as it occurs among sharks gener- 
ally and among our common skates. The jaw of our large barn- 
door skate, Raja ocellata, for instance, exhibits vertical rows of 
teeth placed one behind the other, as in the sharks, the innermost 
of which are immature, while those along the outer edge of the jaw 
are ready to drop. Soke of these teeth may be so slightly attached 
to the gum that they drop readily, while others are so connected with 
the jaw that they serve their purpose for a longer time. Now in 
Selachians which have this mode of teething the teeth begin to show 
themselves rather late in life. The embryos of these sharks and 
skates do not have teeth, and even after birth the young show very 
imperfect or rudimentary teeth. In some of them, after being for 
some time in the water and providing for their own food, the teeth 
are so imperfectly developed that no row is visible along the edge 
of the jaw ; but when the teeth rise to the margin of the jaw their — 
number. is already fixed and the young teeth are formed in rows 
behind those of the outer series. From that time no other change ' 
takes place except that larger and larger teeth are formed behind 
the old ones as these drop in succession; and as the old ones drop 
the next oldest come into play and so on. 
In Galeocerdo, a genus of much interest to paleontologists, the 
teeth are very uniform in both jaws and more nearly of the same 
size than in other families, while the teeth which are to replace the 
old ones are not much larger, thus showing that these sharks grow 
very slowly. These facts are very important with reference to 
_ the identification of the fossil, species. In younger specimens of- 
this genus there are fewer rows of replacement teeth and they re- 
semble adult teeth much less than they do here. As the jaw en- 
larges with age, the new sets of teeth enlarge, -e and in that 
_ Manner the whole margin is always occupied by tee 
In Cestracion, on the contrary (and der this name a designate 
` the Port Jackson shark and not the hammer-heads), we have a 
n ` totally different mode of teething, the knowledge of which is es- 
sential to a correct PAUE of the zoological value of a vast 
number of f fossil teeth 
ae 
i “i the olden and middle we o 
