Antmal Dentition 
spine. And it is not a little remarkable that, while 
the male of the common Thornback Skate has teeth 
of this type, m the female of the same species all 
the teeth are of the crushing modification. It 
would seem almost certain that this indicates a 
difference in the diet of the two sexes, although 
I have not seen any statement that such really Pee ones Reape te at ies ea ease eE 
astiste the genus Myliobatis. 
Very beautiful is the dentition of the Beaked Rays of the genus Rhynchobatis, 
in which the teeth are small and of a crushing type, arranged quincuncially, the beauty 
of the whole structure being much enhanced by the circumstance that the dental 
surface of the jaws is thrown into undulations, with the prominences of the one fitting 
into the hollows of the other. Yet another type, and one in which the millstone-like 
structure attains its supreme development, is presented by the Hagle-Rays. In one of 
these, Rhinoptera (Fig. 10), the teeth are arranged in from five to nine rows, those of 
the three middle rows forming transversely elongated bars, while the lateral ones are 
hexagonal tessare. In the true eagle-rays of the genus Myliobatis (Fig. 9), on 
the other hand, there are seven longitudinal rows of teeth, of which only the 
central ones are transversely elongated. Finally, in the species of Méobatis all - the 
lateral rows of small teeth have 
disappeared, and only the large 
central series persists. Some of 
the fossil representatives of the 
last two genera, judging from 
the size of their dental plates, 
must have attained enormous 
dimensions, far exceeding those 
of the existing forms, which in 
some cases measure twelve or 
fifteen feet across the fins. 
To extinct rays, probably 
nearly allied to those just men- 
tioned, belonged the beautiful 
teeth from the Chalk similar to the one shown in Fig. 11. The teeth of these 
rays (Ptychodus) vary much in size in the same individual, and from specimens 
showing a number in association, which occasionally come to hand in chalk-pits, it is 
evident that these teeth were arranged in alternating longitudinal rows of large and 
small ones. All these teeth have broad and low crowns, with a marginal area marked 
by small pustules, and an elevated centre carrying a few 
smooth transverse ridges with a variable number of 
pustules between and around them. 
That very remarkable fish the Chimera, or king-of- 
the-herrings, is the type of an ordinal group of fishes 
represented by only one other living genus, but containing 
a number of extinct forms. In these chimeroids, as they 
are collectively called, the teeth are quite unlike those of 
the sharks and rays, and are of very large size, constituting, 
in fact, the whole of the solid portions of the jaws, as 
there are no bones corresponding to those of the modern 
type of ordinary fishes. It is not, however, by any 
Fig. 10. A Dental Plate of a Ray of the genus Ithinoplera. 
; 3 < Fig. 11. Tooth of the Cretaceous 
means the whole surface of these massive teeth that is Ray (Ptychodus). 
