ral form is that of a broad base or plate from which a short spine is
produced ; this is most conspicuous in the central rows of teeth where
the spine is developed from the inner angle, but it gradually diminishes
as the teeth approach the sides of the jaws. The vertical series
of teeth describe nearly a complete circle at the middle part of the
jaws, and include about twenty teeth each.
The teeth of the Torpedo are somewhat stronger, their base is
more extended transversely, and the pointed cusp is more produced
than in the common rays.
In the thornback (Raia clavata), the teeth present the form of
small transversely oval, obtuse masses ; and the vertical series are so
closely arranged, that being alternate, they are less conspicuous than
the oblique rows which result from this approximation. Mr. Yarrell
describes the following sexual character in the teeth of the Thornback
ray. “ While both sexes are young, the teeth in both are alike broad
and flat; but as the male acquires age and sexual power, the teeth
that are nearest the centre begin to alter in form, and become pointed
by an elongation of the internal angle ; all the points being directed
backwards or towards the throat.”(l)
The quincuncial arrangement of the teeth prevails in the different
species of Trygon, or fire-flare, in which the teeth are of very
small size. Their crowns consist of a triangular plate, with the
internal angle or point most produced; this plate is supported on a
hollow pivot.
In the genus Rhina each tooth is supported on a short fang or
pivot, which tapers as it recedes from the crown ; there is a groove
along the posterior part of this pivot, and a perforation on each side;
the crown is lozenge-shaped, convex above, and sculptured with a
series of transverse and slightly undulating and punctate ridges ;
presenting a pattern which somewhat resembles that of the grinding
surface of the comparatively gigantic tooth of the extinct Ptychodus. (2)
The modification of the dentigerous surface of the jaws, and the beautiful
quincuncial arrangement of these teeth are exhibited in Pl. 23, figs.
1, and 2. The middle part of the upper jaw forms a bold prominent
(1) British Fishes, ii, p. 416.
(2) Compare the magnified view of a denticle of Rhina in PI. 23, fig. 3, with the tooth of
Ptychodus latissimus, PI. 17, figs. 1 and 2.
convexity separated by a depression on each side from a lateral and
less produced rising. The contour of the dentigerous surface of the
lower jaw presents depressions corresponding to the eminences above,
and vice versd.
The structure of the tooth of the Rhina exhibits a modification
of the tubular structure, very different from that which characterises
the tooth of the extinct Ptychodus. A representation of this structure,
as displayed in a longitudinal vertical section of the tooth, is given in
PI. 24, which corresponds with that of the teeth of the rays above
described.
The pivot-like base of one of the component teeth of the pavement
of the jaws of the Rhina consists of a bony texture, denser than
that of the ordinary bone of fishes ; it is traversed by undulating, diversely
directed, medullary canals, with concentrically laminated walls ;
numerous wavy calcigerous tubes radiate from the medullary canals,
subdivide in their interspaces, anastomose together, and intercommunicate
with numerous minute calcigerous cells. The structure
of this part of the tooth, we shall find to be similar to that which
pervades the entire tooth in another family of plagiostomous fishes.
The crown or body of the tooth in the Rhina exhibits, however, a
denser structure, and one that approaches nearer to the ordinary
structure of the dentine in the higher vertebrate classes. Some of
the medullary tubes of the base of the tooth open into the remains
of the cavitas pulpi, now closed up below by the ossification of the base
of the tooth. This cavity presents a conical form, with an oval
transverse section. A uniform system of calcigerous tubes radiates
in every direction, from the pulp-cavity towards the periphery of the
tooth, and vertical to that surface ; those which are continued from
its lower part, bend down towards the root of the tooth ; the lateral
ones are continued directly outwards to the sides, and the superior
tubes proceed vertically to the upper surface ; the intermediate
tubes present every gradation of oblique curvature.(1) All the tubes
have a minutely undulating course ; (2) they dichotomize as they
proceed towards the surface, and give off minute branches, generally
at an angle of 45° in every part of their course. These branches
anastomose together, and with rhomboidal calcigerous cells in the
(1) Pl. 24, fig. 1. (2) ii. fig. 2.