526 Mr. Owen on species of 



Labyrinthodon pachygnathus. 



Lotver Jaw. — I now proceed to describe the fossils on which I have founded a second 

 species of Labyrinthodon, and shall commence with a well-preserved portion of the 

 right ramus of the lower jaw (PI. XLIV. figs. 1, 2, 3.). This fossil measures nine 

 inches and a half in length ; the bone is thick and rounded, one inch and a half deep 

 and one inch broad at the posterior fractured part, thirteen lines broad and seven 

 lines deep at the anterior expanded and subdepressed end, which is curved inwards 

 towards the symphysis of the jaw, and which supports two of the great cuspidate or 

 laniary teeth, and the socket of a third. The structure of this lower jaw, which was 

 broken across into two nearly equal portions, was the first object of attention. On 

 the inner side of the anterior moiety it appeared to be strengthened by an opercular 

 piece, in the form of a thin plate, gradually narrowing to a point, and terminating 

 at the beginning of the inward curvature of the ramus (fig. 2.). This was obvi- 

 ously a more complicated structure than exists in the jaws of fishes ; whilst in rep- 

 tiles the inner side of the ramus of the jaw is supported by a distinct plate of bone 

 in both Batrachians and Saurians. This complication, however, is due to a differ- 

 ent structure in the Batrachia from that which obtains in the Sauria. In the 

 lower jaw of the Frog, for example, it is produced by a continuation of the angular 

 element upon the inner side, as high as upon the outer side of the jaw ; whilst in 

 Saurians the inner splint-like plate exists as a distinct piece, to w^hich Cuvier has 

 given the name of ' os operculare.' The question then to be determined was, to 

 which of those structures was due the appearance above mentioned, of an opercular 

 piece on the inner side of the maxillary ramus in the Keuper reptile ? I may first 

 observe, that the outer side of this ramus presents a structure not to be found in any 

 Saurian jaws, viz. a well-marked longitudinal impression running parallel with the 

 lower border of the jaw, about half-way between that and the alveolar border, towards 

 which it gradually inclines, and then abruptly bends downwards and backwards. 

 This impression evidently leads to a harmonia or toothless suture (like those which 

 join together the different elements of a compound jaw), into which the sandy 

 matrix has insinuated itself. The lower piece corresponds in depth and forward 

 production with the large angular element of the jaw of the Frog, in which the an- 

 gular piece is continued nearer to the symphysis than in any known Saurian ; but 

 in the fossil it swells out as it were beyond the level of the upper or dentary piece, 

 and forms a bold and broad convex ridge along the lower part of the outer side of 

 the ramus, which subsides at only a short distance from the symphysial bend. The 

 angular piece forms a similar projection on the outer side of the posterior half of 

 the jaw in the Frog. 



Two transverse fractures of the present fossil display the relations of the external 

 plate of the angular piece with the thin internal bony laminae ; the two pieces are 



