from the Shales of the Northumberland Coal-field. 271 



more than half that length ; but its extremity is not quite 

 perfect. The three imperfect anterior teeth are very much 

 larger than the four posterior ones ; the largest of them, which 

 is the centre one, cannot have been less than an inch in length 

 and three-eighths of an inch wide at the base. They are all 

 placed considerably apart from each other ; but they are not 

 quite so widely separated as those in the other fragment. 



We have two other fragments of jawbones of this reptile ; but 

 they are in such an unsatisfactory condition that not much can 

 be said about them. One of them, however, is probably a 

 maxillary bone ; it is six and a half inches long, and an inch 

 and a half wide ; but it is so imperfect that its form is not 

 determinable, and it is much encumbered with other bones. 

 A portion of the alveolar border is nevertheless distinctly dis- 

 played, with five close-set teeth, the largest of which is up- 

 wards of half an inch long. Other fragments of teeth are 

 scattered about. We have also a confused mass of apparently 

 cranial bones, which show the peculiar surface-sculpture. A 

 portion of a jaw with a tooth or two is mixed up with these 

 fragments. 



For some time these portions of mandibles were all sup- 

 posed to belong to Rhizodus lanceiformis^ Newberry, as the 

 teeth attached to them are imdoubtedly similar to those of 

 that reputed fish. But on considering that the sculpture of 

 the bone-surface is exactly like that of the cranial and pre- 

 maxillary bones, that the teeth of the latter are similar to 

 those of the mandibles, and that the teeth of both exhibit pre- 

 cisely the same Labyrinthodont structure when examined in 

 transverse section, the conclusion that these mandibular frag- 

 ments really belong to this new Labyrinthodont is quite irre- 

 sistible. The piscine nature of R. lanceiformis is therefore 

 questionable. Certainly the teeth usually so designated be- 

 long to this reptile ; and unless other evidence be forthcoming, 

 this reputed species of Rhizodus will have to be erased from the 

 list of Carboniferous fishes. 



The two sets of sternal plates are characterized by the same 

 surface-ornament which we have found on the other bones ; 

 and though neither of them is perfect, sufficient is preserved 

 to enable us, by the aid of both, to form a very good idea of 

 their shape and characters. In the finer and larger specimen, 

 all the three plates are present, and by their position exhibit, 

 though considerably displaced, their relation to each other. 

 The posterior ends of the two lateral plates lie in juxtaposition, 

 overlapping the anterior portion of the central plate, and di- 

 verging backwards ; a portion only of the posterior margin 

 of the central plate is exposed. By removing the matrix, 



