figs. 1 and 2) presents a quadrangular form, and consists of a root
and a crown ; a wide but shallow groove 'forms the line of demarcation
between these parts. The root, which presents a coarse, porous,
osseous texture, is flat below, and gradually enlarges towards the
crown. The crown suddenly expands above and beyond the root,
especially beyond its anterior part. The anterior and two lateral
surfaces of the crown are convex, the posterior surface (fig. 1) is
concave, apparently for the reception of the convexity of the succeeding
tooth. The broad upper surface of the crown is more or less
convex, and sculptured with minute tubercles and wrinkles at the
circumference, and with large angular transverse ridges in the centre.
These ridges are separated by wide grooves, and generally have their
extremities bent towards the anterior part of the tooth ; the surface
of the crown is smooth and polished like enamel. In fig. 2, the
sculptured grinding surface of a tooth of the Ptychodus latissimus is
represented of the natural size.
The texture of the tooth of the Ptychodus, examined in a longitudinal
vertical section at half an inch focus, as in PI. 18, presents a
congeries of medullary and calcigerous tubes, having the same general
arrangement as in the Cestracion. The medullary tubes are, however,
relatively smaller ; they proceed from the coarse canals of the osseous
base in a nearly straight and parallel direction towards the surface
of the crown, diverging from each other, and branching dichoto-
mously, so as to maintain a direction vertical to the surface towards
which they proceed. Their interspaces are pretty regular, and about
five or six times the diameter of the canals themselves. They are
surrounded by concentric lamelfe (fig. 2, PI. 19), and send off through
the whole of their course numerous minute calcigerous tubes ; these
are transverse to the medullary canals near the base of the crown, and
come off at an acute angle as they approach nearer the summit, close
to the surface of which the medullary canals resolve themselves into
fasciculi of calcigerous tubes (fig. 1, PI. 19).; in a few places, two
contiguous medullary canals anastomose, and form a loop, with the
convexity directed towards the surface of the tooth ; one of these
loops is shown in fig. 1, PI. 19. The calcigerous tubes are more
I Wavy than the medullary canals ; they quickly ramify, and sub-divide
I to extreme minuteness in the interspaces, and finally terminate by
I anastomosing with each other, either immediately or by the interposition
of calcigerous cells.
The intimate structure of the tooth of the Ptychodus differs from
that of the tooth of the Cestracion, in the medullary tubes being
narrower, the interspaces wider, and the terminal anastomosing loops
fewer. The calcigerous tubes, also, are relatively larger, more wavy,
and more branched. It differs from the structure of the tooth of
the Acrodus in the straighter course, and fewer divisions of the medullary
canals, and in the absence of the straight parallel superficial
series of calcigerous tubes.
22.Psammodus.—Under this name,M. Agassiz had formerly associated
all those teeth of fishes “ which combine a structure like that
which characterizes the teeth of the Cestracion,” that is to say, a crown
formed of small vertical tubes,—“ with a surface of the crown more or
1 less smooth, and presenting only that sanded or punctate character
I which results from the structure of the crown.”
Here, however, I must observe that neither in the teeth of I Psammodus nor in those of Cestracion have the punctate impressions I of the enamelled surface any relation with the medullary canals or I the large visible vertical tubes to which the learned Ichthyologist just
I quoted refers. These tubes always terminate at a short distance
I from the surface of the tooth, either by anastomosis or by subdivision
I into other tubes of such extreme minuteness that the combined
I diameters of five hundred of them would barely equal the breadth
I of a single superficial punctation. These impressions on the teeth of
I the Psammodi, like the transverse ridges of those of the Ptychodi, are
I consequences of the conformation of the original matrix, and can be
I regarded only as adaptations conformable with the habits and food
I of the extinct species ; and as they are not due to a certain tubular
I structure, so neither can they be viewed as evidence of such structure
I when it has not otherwise been proved to exist.
The term Psammodus is now restricted by M. Agassiz to the
extinct fishes, the teeth of which combine a broad, flat, punctate,