elements of the temporal bone. In its simplicity, in the plagios- I
tomes, it participates with the character of the rest of their cartilagi- I
nous cranium, in which the several distinct bony elements found I
in the reptiles and osseous fishes cease to he recognisable. Of the 1
several bones concerned in the formation of the jaws, we have to I
seek, in the simple cartilaginous arches of the skate, the pterygoid, L!
palatine, maxillary, intermaxillary, mandibular, and premandibular
bones; and to these may be likewise added the labial cartilages, I
which are so largely developed in the cyclostomous fishes. Are all
these elements combined in the dentigerous cartilages of the skate ? I
or if not all, which ? These questions have been differently answered
by different comparative anatomists. The essential character of the I
pterygoid and palatine bones manifests itself, in the oviparous classes, I
in the formation of buttresses extending between the vomer and the I
articular pedicle of the jaw. In the Carcharias glaucus, or blue-shark, I
and in the Lamna, or porbeagle, a distinct flattened process of car- I
til age extends from each side of the vomerine region of the skull, I
and abuts against the proximal extremity of the pedicle, and the I
contiguous part of the cranium. These processes, I regard as I
analogous to the palato-pterygoidean buttresses. In the common I
torpedo, there is a distinct cartilage in the corresponding situation,
and in the Brazilian torpedo,(1) Dr. Henld has discovered a second
broader cartilage, anterior and internal to the pterygoidean pedicle,
and which he considers to be the analogue of the true palatine
bone ; this cartilage has not been found separately developed in any
other plagiostome.
With respect to the labial cartilages these are wanting, according
to Muller, in the following subgenera of the ray tribe, Raia,
Trygon, Rhinobates, Cephaloptera, and Myliobates; they are also
absent in the Carcharias, Cestracion, and Pristis among the sharks. |
They are present in the Tope (Galeus), which has one on each side
of the upper lip; in Scymnus there are corresponding cartilages, which
are elongated and extend below the angle of the mouth; in Scyllium
and Mustelus, there is one on each side of both upper and lower lips; 1
(1) Turpedo braziliensis, the type of the sub-genus Narcine of Muller and Henle'.
in Centrina, and Squatina, there are two on each side of the upper,
and one on each side of the lower lip; these are figured at a 6 and c,
PI. 10, fig. 2, in the monk-fish. In the Narcine or Brazilian torpedo,
distinct labial cartilages are associated with the dentigerous maxillary
arches, and also with the palatine and pterygoidean cartilages ;
; Professor Müller has, therefore, rejected the interpretation of Cuvier,
according to which, the anterior or superior dentigerous arch of the
plagiostomes is the homologue of the palatine bones, and the posterior
one, the homologue of the post-mandibular element of the lower jaw,
the intermaxillary, maxillary, and premandibular bones being repre-
; sented by the edentulous labial cartilages. This interpretation,
besides being invalidated by the anatomy of the Narcine, also involves
the anomaly of the teeth being developed on the articular, or post-
mandibular element of the lower jaw, where they are never situated
in any other vertebrate animal. A more extended comparison than
Cuvier had the means of instituting, and especially a study of the
structure of the cranium of the Cestracion, in which the labial cartilages
have disappeared, and the development of the dentigerous
arches have advanced nearer to the osseous type than in other
plagiostomes, clearly prove that the dentigerous cartilaginous arches
of the sharks and rays represent, the one, the combined maxillaries
and intermaxillaries, the other, the confluent articular and dentary
elements of-the lower jaw.
The teeth are not immediately connected with these cartilaginous
arches ; no cartilaginous fish has teeth implanted in maxillary alveolar
cavities, or confluent with the substance of the jaw even when
the external crust is ossified, but they are always attached as already
stated, to the fibrous and mucous membranes which cover the maxillary
cartilages ;(1) hence, it occurs in certain genera, as Myliobates and
Scymnus, that a single tooth in the median plane may lie directly
across the symphysis, and be supported by the two rami of the jaw.
The plagiostomes, like many other natural families of fishes, pre-
(1) Any organic fossil which exhibits a tooth implanted by two fangs in a double
socket must be mammiferous, since the only fishes’ teeth which approach such a tooth in form
are those with a bifurcate base, belonging to certain sharks, while the socketed teeth of reptiles
have only a single fang.