250 Animal Life 
each tooth, which finally assumes a distinctly 
cuspidate form in the teeth at the apex of the 
jaw. Evidently this type of dentition is intended 
for a diet very different from that of ordinary 
sharks, which feed by tearing off large masses of 
flesh and swallowing them whole. As a matter of 
fact, the Port Jackson shark feeds chiefly upon igs 4 Dental Plate of a Sharp-Toothed Ray. 
shelled molluscs, for crushing which the pavement- 
like teeth of the hinder part of the jaws are admirably adapted. Crushing-teeth of 
a type very similar to those of the Port Jackson shark are very common in the 
Oolites and Iiias, but, as already mentioned, are of much larger size, many of them 
measuring fully an inch and a half across. Before leaving this part of my subject, 
attention may be directed to the extreme beauty of the dentition of the Port Jackson 
shark, especially the crushing portion. A veproduction of this nught, I think, be 
introduced with excellent effect into ornamental architecture. 
Nearly allied in structure to the hinder crushing-teeth of the Port Jackson shark 
are those of the Carboniferous genus Orodus, two of which are shown in Fig. 6. 
The name Orvodus signifies “ mountain-tooth,” and has been applied to the members 
of this group owing to the rugged and centrally-clevated form of the teeth. 
Before leaving the dentition of the sharks, a few lines must be devoted to 
certain very remarkable extinct types which display some of the most beautiful 
spirals to be met with in nature. The best-preserved specimens are obtained from 
strata—corresponding roughly in age with the upper portion of ow: own Carboniferous 
system-—in the Government of Perm, Russia, and the sharks to which they belonged have 
been named Helicoprion. One of these specimens is shown in the accompanying 
figure (7). It is supposed to be a single transverse row of teeth from either the 
upper or lower jaw. During life the individual teeth seem to have been continually 
replaced from within in the usual manner, while those at the top of the arched 
series would be in use. ‘he discarded teeth did not, however, fall away from the jaw 
after the manner of those of modern sharks and rays, but remained fixed together 
by their bases, so that the series thus formed gradually curved into a complex spiral, 
which was either buried in or overhung by the lip. A similar mode of growth 
existed in certain contemporaneous sharks (cochliodonts) with teeth of a crushing instead 
of a piercing type. 
Not less interesting and beautiful than those 
of the sharks are the types of dentition presented 
by the numerous kinds of skates and rays, both 
living and extinct—although exigencies of space 
admit of reference only to a few. As a rule, the 
dentition of the latte: fishes 1s of the crushing 
type, and the “pavements” formed by these 
crushing-teeth are even more solid than those 
of the Port Jackson shark. Unlike those of the 
sharks, which are always arranged on the sides 
of the jaws in oblique rows, the teeth of the 
rays invariably form straight longitudinal series, 
although, by the alternation of the individual 
teeth, a quincuncial arrangement may also be 
produced (Fig. 9). In some cases, as in Fig. 8, 
Fig. 7. Dentition of Spiral-Poothed Shark the individual teeth are very numerous, of small 
(Helicoprion). 9 F 5 ° G 
[The details of the central portion are omitted.] SIZE, and terminate in a short recurved cusp, or 
te 
