of Wirtemberg and Warwickshire. 507 



constant in and characteristic of each genus or species, is always more or less ir- 

 regular and unsymmetrical in itself. 



There is no instance in the Mammahan class of these folds converging at regular 

 intervals from the whole of the circumference of the tooth towards its centre ; such 

 a disposition of the external substance of the tooth may be traced at the base of 

 the tooth in a few fishes, but in reptiles has hitherto been met with in the fang of 

 the Ichthyosaurus alone. 



In this extinct Saurian the external layer of cement (for the enamel ceases at the 

 base of the crown) is inflected at pretty regular distances around the circumference 

 of the tooth towards its centre, the vertical folds being straight and plane, and ex- 

 tending into the substance of the tooth to a distance about equal to the breadth 

 of their interspaces. These interspaces are occupied by corresponding processes 

 of the dentine, which radiate or diverge from the central mass of that substance. 



If we could suppose the tooth of the Ichthyosaurus to be worn down in the living 

 animal as far as its complicated basis, then an eighth part of the diameter of the 

 tooth around its circumference would present a series of ridges of the denser sub- 

 stance converging in straight lines from that circumference towards the central 

 pulp-cavity. 



The plan and principle of the structure of the tooth of the Labyrinthodon are the 

 same as those of the tooth of the Ichthyosaurus, but it is carried out to the highest 

 degree of complication. The converging folds of the external cement, (Cut 1, b, b.) 

 instead of being arrested at one-fourth of the distance from the circumference to 

 the centre of the tooth, are continued close to that centre ; and instead of con- 

 sisting of simple, straight, and plane lamellae, they are bent upon themselves in a 

 series of irregular folds resembling the labyrinthine convolutions of the surface of 

 the brain. 



The ordinary laws of the complication of dental structure are here, however, 

 strictly adhered to, and every space intercepted by a convolution of the converging 

 folds of the cement is occupied by a corresponding process of the diverging layers 

 of the dentine, and thus is produced the singularly complicated appearance which 

 a transverse section of the tooth of the Labyrinthodon exhibits. 



The external longitudinal flutings of the base of the tooth of the Ichthyosaurus 

 are much coarser, and more indicative of the converging vertical folds of the cement 

 than are the corresponding longitudinal lines on the exterior of the tooth of the 

 Labyrinthodon, — a difference which is owing to the layer of the inflected cement 

 being much thicker in the Ichthyosaurus. 



The external strise of the Labyrinthodon' s tooth are thus of a nature to attract 

 but little attention, and could not have been suspected to be the lines of inflection 



