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370 DENTAL ANATOMY.

In tlie Plagiostomi (sharks and rays), on the other hand, a separate car-
tihiginous element representing the hyo-mandibuku* bone is always pres-
ent, and affords an articular surface to the lower jaw or mandible, which,
moreover, in all the ela.smobranchiates consists of a single cartilaginous
bar, the primitive Meckelian cartilage. The palato-quadrate arch is
likewise present and forms the dentigerous border of the upper jaw (see
Fig. 204). Since the hyo-branchial skeleton in these forms is not con-
cerned in the support of teeth, it can be dismissed without further
mention.
The principal deviations in the structure and relationship of the den-
tigerous apparatus from the typical teleostean one to be met with in the
sub-class Ganoidei are furnished by the Dipnoi and Chondroganoidei.
The former of these orders includes the three living genera C'eratodus,
I-'rofopfcrus, and Lepidosircn of Australia, Africa, and South America
respectively. They are most remarkable and interesting representatives
of types in some respects low down in the scale of ichthyic organization,
while in others high, in that they furnish many transitional characters
between true fishes and the Batrachia (frogs and salamanders). It is
highly probable that from some as yet undiscovered relative of this
group the Batrachia have been derived by descent.
In this order the skull is devoid of both maxillae and prcmaxillse,
and, as in the chimeroid elasmobranchiates, the hyo-mandibular arch is
not completely differentiated, the lower jaw being articulated directly to
the cranium. There is, hoAvever, a well-defined palato-quadrate arch
supporting teeth. The hyo-branchial skeleton, although resembling the
teleostean type of structure considerably, is edentulous. In the L'hon-
droc/anoidci (sturgeons) the skull as well as the arches remain largely
cartilaginous. The suspensorium (proximal part of the hyo-mandibular
arch) presents two elements, usually homologized with the hyo-mandib-
ular and quadrate pieces of the teleostean skull ; the latter of these
pieces affords attachment to the mandible. There is also a palato-quad-
rate arch. Only one species of this group, the shovel-nose sturgeon,
possesses teeth, and these, according to Owen, apjiear only in the young.
The remainder of the Ganoidei agree with the Telcostci in the structure
and arrangement of the accessory organs. The latter sub-class, how-
ever, exhibits numerous minor variations, which are confined principally
to modifications of the hyo-branchial skeleton, such as the loss or atrojihy
of certain of its component elements; these are so numerous and varied
in their nature that it would be impossible, and quite foreign to the
object of the present article, to enumerate them.
Teetli of the Elaxmohranehii.—As already observed, this class is divis-
ible, not only by the differences which obtain in the arrangement of the
several arches, but by the disposition, structure, and manner of replace-
ment of tlie teeth, into two ])rimary groups, of which the sharks
and ravs constitute one, and the Chiniferce the other. Of these, the
former is the more primitive, and in all probability gave origin to the
typical fishes, while the latter rcseml)les more closely the dipnoans, and
may indeed })rove to liave been their ancestors.
The teeth of the sharks are always numerous, and are ]H^e-eminently
adapted to the predaceous habits of their possessor. They are borne