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374 DENTAL ANATOMY.
are more closely set, but are not in absolute contact ; as in Baja stelluata,
those of the several rows are arranged vertically, but their bases are
more rounded, with only a faint indication of the backwardly project-
ing cusp, which is confined to the teeth of the anterior part of the jaws.
In the common "stingray" {Trygon centruriis) the teeth are some-
what quadrangular, and have their sides directly applied to each other,
forming a dental sheath of continuous pavement over the working sur-
face of the jaws; those of the successive rows are disposed diagonally.
Their crowns are of an oval form, well adapted for crushing and grind-
ing hard substances. The " eagle rays" or "sea-devils" present a series
of modifications of the teeth which diverges from that of the stingrays,
and terminates in the most unique of all dentitions to be found amongst
the Vertebrata—viz. that of Aetohaiis. Of this group the genus Bhi-
noptera possesses tessellated teeth with flat hexagonal crowns, of which
Fig. 193.

h c
c, Mylidbatis; d, Aelobatis.
Teeth of Rays: a, b, Rhinoptera ;
the median or anterior ones may be elongated transversely. The fossil
species, M. Woodwardi, has the three median vertical rows enlarged.
In llyliobatis there is only one large median row, Mith three smaller
ones upon either side, M'hile in Aelobatis the teeth of the median row
alone remain, and are articulated to each other by a finely serrate border.
These modifications are well shown in the accompanying figures. The
anomalous saw^fish (Pnstis), although in no way peculiar as far as the
teeth of the mouth are concerned, nevertheless j)ossesses a remarkably
elongated snout, armed u})on either side by a row of hard, conical liodies
usually referred to as teeth. In their histological structure they agree
with true teeth, but exhibit the peculiarity of being lodged in separate
sockets and growing from persistent pulps—a condition unusual among
fishes. It is more than probable that they are dermal spines specially
developed in this situation for some important purpose which is not at
present fully determined.