FAMILY I.- CESTRACIONTIDIE. (Port-Jackson Shark.) The existing genus which has thrown most light upon the fossil teeth which have thus become imbedded in the oceanic deposits of the paheozoic and mezozoic periods, is the Cestracion, now restricted to the Australian and Chinese seas, where it is represented by two or three species, and suggests the idea of a form verging towards extinction. It formerly flourished under a great number of varied generic or family modifications, presented by species, some of which attained dimensions far exceeding the largest known living Cestracions. The tion of these fishes is adapted to the prehension and tion of crustaceous and testaceous animals ; they are of a harmless, timid character ; and have the before-described ticulate dorsal spines given to them as defensive weapons. Fig. 27 gives a side view of the upper and lower jaws of the " Port Jackson shark," showing the oblique tion of the large crushing teeth, which cover like a pavement the working borders of the mouth. The anterior teeth were small and pointed. Behind the cuspidate teeth the five consecutive rows of teeth progressively increase in all their dimensions, but principally in their antero-posterior extent. The sharp point is converted into a longitudinal ridge traversing a vex crushing surface, and the ridge itself disappears in the largest teeth. As the teeth increase in size, they diminish in number in each row. The series of the largest teeth includes from six to seven in the upper, and from seven to eight in the lower jaw. Behind this row the teeth, although preserving Upper jaw and teeth of Port-Jackson Shark (Cestracion), half nat. size.
their form as crushing instruments, progressively diminish in size, while at the same time the number composing each row decreases. From the oblique and apparently spiral disposi tion of the rows of teeth, their symmetrical arrangement on the opposite sides of the jaw, and their graduated diversity of form, they constitute the most elegant tesselated covering to the jaws which is to be met with in the whole class of fishes.
The modifications of the form of the teeth above described, by which the anterior ones are adapted for seizing and re taining, and the posterior for cracking and crushing alimentary substances, are frequently repeated, with various modifications and under different conditions, in the osseous fishes. They indicate, in the present cartilaginous species, a diet of a lower organized character than in the true sharks ; and a correspond ing difference of habit and disposition is associated therewith. The testaceous and crustaceous invertebrate animals constitute most probably the principal food of the Cestracion, as they appear, by their abundant remains in secondary rocks, to have done in regard to the extinct Cestracionts, with whose fossil teeth they are associated.
From their mode of attachment, these teeth would become detached from the jaws of the dead fish, and dispersed in the way above described ; and it is by such detached fossil teeth that we first get dental evidence of the Cestraciont family in former periods of the earth's history.
The teeth of the Hybodonts are conical, but broader and less sharp than those of true sharks. The enamel is strongly marked by longitudinal grooves and folds. One cone is larger than the rest, and called the "principal ;" the others are " second ary." In one genus (Cladodus, Ag.), the secondary cones go on enlarging as they recede from the principal cone ; and teeth of this genus, referred by Eichwald to the Hybodus longi conus, have been discovered in the old red sandstone in the vicinity of Petersburg.