216 LABYRINTHODONTS. 



where the lobes divide, and terminate each in a dilated cavity, or 

 medullary canal, about half way between the centre and the circum- 

 ference of the tooth. In like manner the inflected folds of cement 

 (&, b), converging from the circumference of the section, proceed 

 inwards in a straight line, half way or three fourths of the way to the 

 central pulp-cavity ; the commencement of the lateral inflection of one 

 of the longest of these folds is shown at b. The inflected substance con- 

 sists of a very thin layer of cement. The calcigerous tubes radiate 

 according to the usual laws, and resemble in diameter, in the 

 width of their interspaces, in their secondary undulations, their 

 dichotomous bifurcations and small lateral branches, the same 

 tubes in the complex labyrinthic structure of the tusk, figured in 

 Plate 64 a. The exterior or first formed part of the basal portion of 

 the tooth resembles the above described apical part in the compara- 

 tive simplicity of the alternate folds of cement and processes of den- 

 tine ; but the central mass of the basal moiety of the tooth presents 

 the labyrinthic disposition of these tissues : it may be said that the 

 simple exterior crust of the tooth incloses and is expanded by the 

 more comphcated structure. The thickness of the exterior more 

 simple structure is nearly the same in the serial teeth and the large 

 tusks, so that the proportion of the central labyrinthic mass is greater 

 in the latter, and its structure is likewise more complicated. In the 

 serial teeth of the Lab. pachygnathus, the structure of the basal half 

 of the tooth resembles that of the same part in the Lab. leptognathus, 

 which is figured as seen in transverse section at PL 63 b, fig. 2. 



The alveoh, in both upper and lower jaws of the Lab. 

 pachygnathus are large, moderately deep, but complete. The outer 

 wall of the alveolar processes in the lower jaw of the Lab. pa- 

 chygnathus is not higher than the inner wall. In this structure, 

 therefore, as well as in the division of the alveolar groove into 

 sockets, the Labyrinthodon manifests an affinity to the Crocodihan 

 and Pleriosaurian reptiles ; but, on the other hand, a similarly com- 

 plete dental socket is present in certain Scomberoid and Sauroid 

 fishes ; and, as in these fishes, the base of the tooth is anchylosed to 

 the socket in both the Labyrinthodon pachygnathus and Lab. lepto- 

 gnathus. A still more striking Ichthyic character is manifested by the 

 Lab. pachygnathus in the continuation of the row of small teeth 



