family, Hybodonts, beginning with the Muschel-kalk, and perhaps with the Coal formation, prevails throughout the Oolite series, and ceases at the commencement of the Chalk. The third family of "Squaloids," or true Sharks, commences with the Cretaceous formation, and extends through the Tertiary strata into the actual creation.[1]
- ↑ The character of the Cestracionts is marked by the presence of large polygonal obtuse enamelled teeth, covering the interior of the mouth with a kind of tessellated pavement. (Pl. 27d. A. 1, 3, 4, and Pl. 27d, B. 1, 2, 3, 4, 5.) In some species not less than sixty of these teeth occupied each jaw. They are rarely found connected together in a fossil state, in consequence of the perishable nature of the cartilaginous bones to which they were attached; hence the spines and teeth usually afford the only evidence of the former existence of these extinct fossil species. They are dispersed abundantly throughout all strata, from the Carboniferous series to the most recent Chalk.
In plate 27e, Figs. 1, 2, represent a series of teeth of the genus Acrodus, in the family of Cestracionts, from the lias of Somersetshire; and Pl. 27f, a series of teeth of the genus Ptychodus, in the same family, a genus occurs abundantly and exclusively in the Chalk formation.
In the section Pl. 1, Fig. 19 represents a tooth of Psammodus, and Fig. 19', a tooth of Orodus, from the Carboniferous limestone; and Fig. 18", a recent tooth of the Cestracion Philippi. The Cestracion Philippi, (Pl. 1, Fig. 18, and Pl. 27d, A.) is the only living species in the family of Sharks that has flat tessellated teeth, and enables us to refer numerous fossil teeth of similar construction to the same family. As the small anterior cutting teeth (Pl. 27d, A. Figs. 1. 2. 5.) in this species, present a character of true Sharks, which has not been found in any of the fossil Cestracionts, we have in this dentition of a living species, the only known link that connects the nearly extinct family of Cestracionts with the true Sharks or Squaloids.
The second division of the family of Sharks, Hybodonts, commencing probably with the Coal formation, prevailed during the deposition of all the Secondary strata beneath the Chalk; the teeth of this division possess intermediate characters between the blunt polygonal crushing teeth of the sub-family Cestracion, and the smooth and sharp-edged cutting teeth of the Squaloids, or true Sharks, which commenced with the Cretaceous formations; They are distinguished from those of true Sharks by being plicated, both on the external and internal surface of the enamel. (See Plate 27d. B. Figs. 8, 9, 10.) Plate 27d C. 1re. represents a rare example of a series of teeth of Hybodus reticulatus, still adhering to the cartilaginous jaw bones, from the Lias of Lyme Regis. Striated
