PLAGIOSTOMI. 



125 



furmosas (Ag.) The foregoing formations constitute the upper- 

 most of the palaeozoic series called "Permian," from the Eussian 

 province in which these strata are most extensively developed. 

 The superimposed strata marked " trias " in the " Table of 

 Strata/' fig. 1 , p. 5, include also a varied series of red and white 

 sands, marls, and conglomerates, forming collec- 

 tively the " New Bed," or Triassic system. The 

 triassic ichthyodorulites are referable to the genera 

 Kemacanthus, Leiacanthus, and Hybodus. In the 

 " lias," which is the oldest or lowest of the great 

 " oolitic" system, large dorsal spines of Hybodus 

 reticidatus, also spines of Hybodus medius, and 

 Hybodus pyramidalis are found : this genus, how- 

 ever, is represented by detached teeth in the 

 " bone-bed" and in the keuper and muschelkalk 

 members of the "trias." The lias formations give 

 evidence that the dorsal spines and fins of 'Hybo- 

 dus were two in number ; and the genus is shewn, 

 both by the structure of the spme and the form 

 of the teeth, to have had its nearest affinities 

 with the Cestracion amongst existing Plagios- 

 tomes. Hybodus continued to be represented by 

 successive and varying specific forms up to, and 

 including, the cretaceous period. It is therefore 

 a genus of cartilaginous fishes eminently charac- 

 teristic of the secondary or mezozoic period in 

 pala3ontologv, and ranges through every forma- lg * ■ ' 



. . Hybodus sub 



tion of that period. The specimens selected for carinatus. 

 the illustration of the dorsal spine of Hybodus is (Wealden.) 

 that of the H. subcarinatus, from the \Vealden of Tilgate 

 Forest (fig. 39). 



Large fossil spines, longitudinally grooved, have been 

 found associated with the teeth of the extinct, raio-cestraciont 

 genus (Ptychodus) of the chalk formations. 



