THREE SUB-FAMILIES. 219 



family, Hyhodonts, 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.* 



* The character of tlie Cestracionts is marked by the presence of large 

 polygonal obtuse enamelled teeth, covering the interior of the mouth witli 

 a kind of tessellated pavement. (PI. 27'*. A. 1, 3, 4, and PI. 21\ 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 v/hich 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 Corboniferous scries to the most recent 

 Chalk. 



In plate 27«, Figs, 1, 2, represent a scries of teeth of the genus Acrodus, 

 in the family of Cestracionts, from the lias of Somersetshire ; and PI. 27^, a 

 series of teeth of the genus Ptychodus, in the same family, a genus which 

 occurs abundantly and exclusively in the Chalk formation. 



In the section PI. 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 Phillippi. The Cestracion Phillippi, (PI. 1, 

 Fig. 18, and PI. 27^, 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 (PI. 27'*, A. Figs. ]. 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, Hyhodonts, commencing pro- 

 bably with the Coal formation, prevailed during the deposition of all the 

 Secondary strata beneath the Chalk; the teeth of this division possess inter- 

 mediate 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 f jrmations. 

 They are distinguished from those of true Sharks by being plicated, both 

 on the external and internal surface of the enamel. (See Plate 27^. B. Figs. 

 8, 9, 10.) Plate 27*C. l''^ 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 teeth of this family abound in the Stonesfield 

 slate and in the Wealden formation. 



