A LOWER JAW OF LABYRINTHODON, 279 



the articulation (PL XIX. fig. 2). This cavity has two perforations. 

 There is one long narrow superior perforation,which looks upward and 

 extends for more than 4 inches in front of the articulation ; it is mar- 

 gined externally by the surangular bone, internally, presumably, by 

 the splenial. The internal lateral perforation is much smaller ; its 

 place is from 7 to 8 inches in front of the hindmost extremity of the 

 jaw, and half an inch from its inferior border. Its lower border is 

 made by the splenial bone (fig. 2 a), and its upper border probably 

 by the surangular (fig. 2 d), unless a small coronoid were present. 

 The specimen docs not show whether the perforations communicated 

 or were separate. The internal cavity, like the jaw itself, is some- 

 what compressed from side to side behind on its inferior border, and 

 widens a little anteriorly ; it widens more from below upward. It 

 is straight and in the same plane. 



All the bones unite by sagittate sutures, and scarcely show any 

 indications of squamous overlap. They are all more or less seen on 

 the external aspect of the jaw (fig. 1), except the splenial bone ; while 

 every bone is seen on the internal aspect (fig. 2), where all the bones 

 are free from sculpture. The internal aspect is largely occupied by 

 the splenial element (fig. 2 a), which is fractured at its anterior ex- 

 tremity, and along all its upper border behind the lateral perforation. 

 As preserved, it is 11 1 inches long, and was probably some inches 

 longer. It terminates behind by a suture inclined slightly backward, 

 but nearly vertical where it joins the articular bone (fig. 2 e) ; and it 

 rests upon the angular bone (fig. 2 c) below, which extends along this 

 aspect of the jaw for 7| inches. This suture with the angular bone 

 has the outline of a low hill, of which the summit is at 3} inches 

 from the hinder extremity, and the height about 1J inch. Hence 

 the splenial bone descends in position as it passes forward, and then 

 for 1^ inch it crosses obliquely on the underside of the jaw, and so 

 appears on its exterior aspect for 4| inches, extending forward as a 

 very narrow strip, not a quarter of an inch deep (fig. 1 a), which 

 has the angular bone above it for 2\ inches behind, and the dentary 

 bone above it in front. It is strongly marked on the flattened 

 underside of the jaw with a radiate ornament reaching through 

 half a circle. The ornament extends from a centre on the inner 

 margin of the bone, half an inch below an oblong vascular perfor- 

 ation on the inner aspect ; around the centre are close-set pits ; but 

 further away these are elongated into grooves. Every pit and groove 

 has a perforation or channel for one large blood-vessel. 



Of the dentary bone only a small posterior part is preserved (fig. 1 b). 

 It extends backward on the upper surface of the jaw to within 2 

 inches of the articulation, and 4 inches of the extremity of the heel, 

 terminating in a point. It rapidly widens and deepens.^ The surangu- 

 lar bone, over which it extends by squamous overlap, is internal to it 

 on the superior aspect. Externally it descends down the side of the 

 jaw by a long oblique suture, resting at first on the surangular bone 

 for 1| inch, then on the angular bone for 5-J inches, and finally upon 

 the splenial bone. The dentary differs from the other external bones 

 in being smooth and free from ornament, except faint longitudinal 



