306 Geological Society. 



the outer wall of the shallow socket, there being no alveolar ridge 

 external to it. The large anterior fang is three times the size of the 

 first of the serial teeth, and the size of these gradually diminish as 

 they are placed further back; the length of the common-sized being 

 about two lines, and the greatest breadth one-third of a line. The 

 apical two-thirds of each tooth is smooth, but the basal third is fluted, 

 and anchylosed to the outer wall of the socket. The breadth of the 

 upper jaw, opposite the middle of the dental series, was two inches 

 six lines ; in proceeding backwards the jaw gradually expands to 

 three inches, and in proceeding forwards narrows, but in a less 

 degree towards the anterior extremity, and then slightly widens or 

 inclines outwards on account of the large tusks. Where the upper 

 jaw is entire, a portion next the median suture, four lines in breadth, 

 is separated from the maxillary bone by a longitudinal harmonia, 

 and corresponds with the position of the nasal bone in the Crocodile. 

 On comparing the structure of the cranium of the Labyrinthodon 

 with the Batrachian condition of the same part, Mr. Owen shows 

 that an important difference will be found to exist. In both the 

 caducibranchiate and perennibranchiate species, the upper maxillary 

 bones do not extend hoiizontally over the upper surface of the skull, 

 but leave a very wide interval between the maxillary and nasal 

 bones ; and the palatal processes of the former contribute as little to 

 form the floor of the nasal cavity: in the Crocodiles, on the contrary, 

 the palatal processes of the maxillary bones extend horizontally in- 

 wards, and meet at the middle line of the roof, forming an unbroken 

 floor to the nasal cavity. In the Labyrinthodon the superior max- 

 illary bones, as already shown, extend inwards to the nasal bone, 

 constituting with it a continuous roof to the nasal cavities ; but the 

 palatal processes, instead of reaching to the middle line, as in the 

 Crocodiles, are very narrow, as in the Batrachia. The osseous roof 

 of the mouth is principally composed of a pair of broad and flat 

 bones, analogous to the divided vomer in Batrachia, but of much 

 greater relative extent, approaching, in this respect, those of the 

 Menopome, and defending the mouth with a more extensive roof of 

 bone than exists in any Lacertian reptile : " physiologically, there- 

 fore," observes Mr. Owen, " the Labyrinthodon, in this part of its 

 structure, comes nearest to the Crocodile ; but the structure itself, 

 morphologically, is essentially Batrachian." In the Menopome and 

 gigantic Salamander, a row of small teeth extends transversely 

 across the anterior extremity of the vomerine bones : and the occur- 

 rence in the Labyrinthodon of a similar row, consisting in each 

 palatine bone of three median small teeth and two outer larger ones, 

 marks most strongly its Batrachian nature ; and from the outer- 

 most tooth a longitudinal row of small and equal-sized teeth is con- 

 tinued backward along the exterior margin of the palatine bone. 

 The whole of this series of palatal teeth is nearly concentric with 

 the maxillary teeth. 



In Lacertine reptiles the examjjles of a row of palatal teeth are 

 rare, short, and situated towards the back of the palate, upon the 



