528 Mr. Owen on species of 



caused by new inflected folds of the cement near the basis of the tooth ; the apical 

 half of the tooth has a smooth and polished external surface ; the pulp -cavity is 

 continued of small size into the centre of this part of the tooth. In the smaller 

 serial teeth, which in other respects, except their less gradual diminution of size, 

 correspond with the preceding description of the anterior larger tusks, the central 

 pulp-cavity is more quickly obliterated : the texture of the teeth is dense and brittle ; 

 the alveoH, as already described, are large, moderately deep, but complete. The 

 outer wall of the alveolar processes in the present fossil is not higher than the inner 

 wall, as is the case in Frogs and Toads, the Salamanders and the Menopome, in all 

 which Batrachians the base of the teeth is soldered to the inner side of an external 

 alveolar plate, as in the Lacertians. It may be said, therefore, that in the more 

 complete structure of the socket the Labyrinthodon manifests an affinity to the 

 Crocodihan and Plesiosaurian reptiles ; but, on the other hand, a similarly complete 

 dental socket is present in certain Scomberoid and Sauroid fishes. The base of the 

 teeth, moreover, in the Labyrinthodon pachygnathus, as in the Lab. leptognathus , is 

 anchylosed to the bottom of the socket, as is likewise the case in the fishes cited. 

 We have a still more striking ichthyic character in the Labyrinthodon, in the con- 

 tinuation of the row of small teeth anterior and external to the two larger teeth. 

 For a double series of teeth, thus occasioned, does riot exist in the maxillary bones, 

 either superior or inferior, of any Saurian reptile ; but in the Batrachian order it is 

 found in the lower jaw of the Ccscilia, and it is not an uncommon structure in fishes. 

 We cannot notice without interest the manifestation of another ichthyic character 

 in the primaeval Batrachia whose remains we are attempting to interpret*. 



Upper Jaw. — The second fossil of the Labyrinthodon pachygnathus which I have to 

 notice is a fragment of the superior maxillary bone of the left side (PL XLIII. fig. 4.) , 

 three inches and a half in length, including twenty-four of the serial teeth ; these 

 teeth resemble those of the lower jaw, being moderately short, with a thick antero- 

 posteriorly subcompressed, finely- striated base, and a sharp, subincurved apex. 

 The outer alveolar plate is rather deeper than the inner one, but the teeth are im- 

 planted in distinct, though wide and shallow, elliptical sockets. The labyrinthic 

 structure is confined to the striated base : in the upper half of the crown it is as 

 simple as in the Ichthyosaurus or Crocodile, but there is no true enamel : the 

 outer coat consists of a thin layer of cement. The relation of the cement to the 

 dentine, as exhibited by a transverse section of the tooth, one line above the 

 socket, is well calculated, from its greater simplicity, to illustrate the prin- 

 ciple of the more complicated modifications of the labyrinthine dental structure 

 first discovered. The processes which radiate from the pulp-cavity, twelve in 

 number at the line of section, proceed straight to about midway between the 



* The successional teeth in Plesiosaurus and Nothosaurus are sometimes so far developed before they 

 displace their predecessors, as to cause the appearance of a double row. 



