Page 363 - My FlipBook

TEETH OF THE VERTEBRATA. 373

many other dentitions do, the reasonableness of the view originally pro-
posed by J. A. Ryder, to the eliect that mechanical causes have been
largely instrumental in bringing about the modifications of the teeth.
It will be readily understood that the greatest mechanical advantage
would be gained and the greatest pressure exerted by passing the mor-
sel to be crushed to the posterior part of the mouth. The teeth in this
situation or in its vicinity have sustained the greatest amount of strain,
and are consequently most modified, while those of the anterior part of
the mouth have been largely exempt from such influences, and are there-
fore little modified. I will have occasion to recur to this hypothesis on
a future page.
The teeth of the rays present quite as great, if not a greater, range of
variety than do the sharks. In general, they are more numerous, more
closely crowded together, and possess forms better adapted for crushing
than for seizing and lacerating. They are developed in the same way as
in sharks, rising up from the bottom of a thecal fold on the inner sur-
face of the jaw and being carried upward by a rotation outward of the
Fig. 192.

Lower Jaw of Port Jackson Shark {Cestracion phiUippsi).
membrane in which they are imbedded. In Raia stelluata, from the
California coast, the teeth succeed one another vertically, as in Lamna
among the sharks, and do not form a close pavement on the biting sur-
face of the jaws, they being separated from each other by slight intervals.
In form the base of the crown represents an equilateral triangle, with
the apex directed forward; from this a prominent ridge passes back-
ward across the middle line of the base, and is produced into a sharp
conical point. The teeth of the anterior part of the mouth are the
largest, and gradually decrease in size as the canthus or angle of the
mouth is reached. In the " barndoor skate " (Eaia Icevis) the teeth