canals approach the apex of the tooth. The calcigerous tubes
are characterized by their frequent branching and inosculation;
the branches go off generally at right angles to the trunk, which
they nearly equal in size; these quickly anastomose and again
send off smaller branches which similarly anastomose with others
of corresponding size, until the terminal tubes are, for the most
part, lost in a series of minute calcigerous cells which form the
boundaries of the system of calcigerous tubes developed from
each medullary canal. Some of the terminal' tubes of contiguous
systems anastomose across this boundary. Each of the systems of
calcigerous tubes represents a separate denticle, of a prismatic figure,
exhibiting in transverse section generally a more or less regular hexahedron.
Towards the point where two contiguous medullary canals
inosculate the terminal calcigerous tubes of each system begin to
exhibit more frequent anastomoses, and the boundary line is thus
gradually obliterated: the letters a, b, and c, pi. 9, fig. 2, exhibit two
systems of calcigerous tubes blending together near the point where
the two contiguous medullary canals were about to inosculate.
The reticulate arrangement of the calcigerous tubes is more, and
the radiated one less, conspicuous in the rostral teeth of Pristis than
in the teeth of any other species which I have yet examined. The diameter
of the calcigerous tubes at their origin is g4_th of an inch, their
terminal branches may be traced to the minuteness of jo»th of an
inch.
In the embryo of a Pristis six inches long, to which the umbilical
chord was still attached, I found a series of depressions in the skin
along the margins of the rostral prolongation corresponding in number
and relative position with the future teeth; and at the bottom
of each of these dermal follicles, there was a papilla which formed
the apex of a pulp, whose base had already begun to penetrate the
cartilaginous plate of the rostrum. The pulp had the usual dense
and unyielding external ‘ membrana propria,’ and its apex was covered
by a continuation of the tegumentary follicle of extreme thinness,
but there was no true capsule or enamel organ. The calcareous particles
had not begun to be deposited in the tissue of the pulp.
The teeth of the young specimen, of which the head and saw are
figured in PI. 8, fig. 1, were fully calcified, and except that the number
of medullary canals were fewer, and the proportion of the calcigerous
tubes greater, they were in every respect miniature resemblances
of the teeth of the full grown fish, such as are figured of the natural
size at PI. 8, figs. 3 and 4.
15. Squatina.—In the monk-fish, (Squatina Angelus), which makes
the transition from the sharks to the rays, the teeth are arranged
along the jaws in well defined vertical rows; including six teeth in
the anterior and gradually decreasing to three teeth in the posterior
rows. The margins of the teeth are smooth; a small tubercle projects
from the middle of the outer side of the base ; these characters
serve to distinguish them from the lower teeth of the Carchanas,
which in other respects they pretty closely resemble. It may be
further observed, that there is not that difference in the position of the
first and second teeth in each row which is so conspicuous in most
sharks ; but here, as in the ray tribe, the change of direction of the
apex is very gradual and regular from the innermost to the outermost
tooth : this character, and the anterior position of the jaws are illustrated
in PI. 10, fig. 2.
RAIIDJ3.
16. The teeth of the rays are, in general, more numerous than
those of the sharks ; they have less mobility, are more closely
impacted, and in some cases are laterally united together by fine
sutures, so as to form a kind of mosaic pavement on both the upper
and lower jaws. The Myliobates or eagle-rays, which present the last
mentioned condition-unique in the vertebrate subkingdom, have
large and massive teeth; but in the rest of the present family of
cartilaginous fishes, they are remarkable for their small size, as compared
with those of the sharks. The teeth in some species of rays,
are adapted for crushing, but in others they have the middle or
one of the angles of the crown produced into a sharp point. In
all genera of the ray tribe, whatever the diversity of size and shape
of the teeth, they are placed in several rows and succeed each other
uninterruptedly from behind.
In the common skate, (RaiaBatis), the teeth are smaller, much more
numerous and the vertical series are more closely approximated than
in the Squatina, which is the most ray-like of the sharks ; their gene