Geological Society. 59 



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 without 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 

 ordinary laws of dental structure are, however, strictly adhered to, 

 and every space intercepted by a convolution of the folds of the 

 cement is occupied by corresponding processes of the dentine. 

 These characters were presented by a transverse section of a frag- 

 ment of a tooth of the Labyrinthodon Jaegeri from the German 

 Keuper, which included about the middle third part of a tooth, 

 and Mr. Owen considers that the entire length of the tooth might 

 be 3^ inches, and the breadth at the basis 1 \ inch. 



The external longitudinal grooves, which correspond to the in- 

 flected folds of the cement, extend upwards from the base of the 

 tooth to about three-fourths of its height, decreasing in number as 

 the tooth diminishes in thickness, and disappearing about half an 

 inch from the summit of the tooth. Each fold of cement penetrates 

 less deeply as the groove approaches its termination ; and Mr. Owen 



