316 Geological Society. 



many, -which have hitherto been described, consist of teeth, a frag- 

 ment of the skull, and a few broken vertebrae ; and in the Warwick 

 sandstone of teeth only. In this memoir, therefore, Mr. Owen 

 confines his attention to a comparison of the dental structure of the 

 Continental and English remains. The teeth of the Labyrinthodon 

 Jaegeri (Mastodonsaurus Jaeger i, Meyer) of the Keuper are of a 

 simple, conical form, with numerous fine longitudinal striations ; 

 and the teeth transmitted to Mr. Owen from the Warwick sandstone 

 by Dr. Lloyd, bear a very close resemblance to them. Their external 

 characters not being sufficient to establish either specific or generic 

 identity, Mr. Owen had sections prepared for microscopic examina- 

 tion of portions of teeth of the Labyrinthodon Jaegeri forwarded to 

 him by Prof. Jiiger, and of the English reptile ; and though, from 

 his previous examination of the intimate texture of the teeth of the 

 Plesiosaur, Megalosaur, as well as of the Crocodile, Monitor, and 

 most recent Lacertians, he did not hope to detect such modifications 

 of structure as would obviously mark specific or even generic iden- 

 tity, yet the slices exhibited such decided characters, and those of 

 the German fossils agreed so intimately with the sections obtained 

 from the Warwick specimen, that Mr. Owen was enabled not merely 

 to separate these fossils from all known reptilian animals, but to 

 establish a generic community of character in the Keuper and sand- 

 stone remains. It was not, however, until he had caused sections 

 to be made in various directions, and had studied them attentively in 

 comparison with the teeth of true Saurians, Batrachians, and other 

 animals, that he was enabled to comprehend the principle of the 

 singular cerebriform convolutions which pervade the dental struc- 

 ture of this remarkable reptile. The base of the tooth of the Ich- 

 thyosaurus approaches most nearly in character to the peculiarities 

 of nearly the entire tooth of the Labyrinthodon. It is impossible to 

 convey clearly Avithout illustrations the structure alluded to. It 

 may, however, be stated, that in the fang of the tooth of the Ichthy- 

 osaurus vertical folds of the external layer of cement (the enamel 

 ceasing at the base of the crown) are inflected inwards, at pretty 

 regular distances around the circumference of the tooth, towards 

 the centre to a distance about equal the breadth of the interspaces 

 of the inflected folds ; the interspaces being occupied by correspond- 

 ing processes of the dentine, which radiate from the central mass 

 of that substance. The thickness of this interblended cement and 

 dentine, surrounding the pulp-cavity, is about one-eighth of the dia- 

 meter of the tooth. 



The plan and principle of the structure of the tooth of the Laby- 

 rinthodon are the same as those of the tooth of the Ichthyosaurus, 

 but they are carried out to the highest degree of complication. The 

 converging vertical folds of the external cement are continued close 

 to the centre of the tooth, and, instead of being straight, simple la- 

 mellae, they present a series of irregular folds, increasing in com- 

 plexity as they proceed inwards, and resembling the labyrinthic an- 

 fractuosities of the surface of the brain ; each converging fold is 

 slightly dilated at its termination close to the pulp-cavity. The 



