FATJEA OF THE GOSATJ FORMATION. 



639 



spaces. The sockets were circular, and indicate larger teeth in the 

 fore part of the jaw than in the hind part. The alveoli for twenty 

 teeth are shown (fig. 9) ; they did not reach to the extremity of the 

 jaw ; nor apparently was there any bony union between the rami ; but 

 the small fragment anterior to the termination of the alveolar 

 margin is broken away. The length of the alveolar margin is 

 about 8 centimetres ; and the extreme length of the fragment is 

 under 9 centimetres. The internal aspect of the jaw has at its 

 base a deep groove, which widens from before backward, and 

 passes close to the base of the jaw (fig. 9), though it appears 

 to slightly ascend, and no doubt lodged the opercular bone. 

 The basal margin below this groove is rounded ; the surface above 

 the groove is smooth, and forms an obliquely twisted area, which 

 maintains a depth of about 1^ centimetre, so far as it is preserved. 

 It is very slightly convex in length, but concave from above down- 

 wards, the concavity increasing forward owing to the increasing 

 twist in the bone. The depth of the jaw at the first alveolus is 

 18 millimetres. At the tenth alveolus it is nearly 2 centimetres ; 

 but the depth cannot be given further back, as the base of the jaw 

 is broken away. The hinder part of the inner alveolar border shows 

 indications of a squamous bone having come awey. This would pre- 

 sumably be part of the opercular bone. 



The external surface (fig. 10), as already remarked, is traversed, 

 at least in part, by a strongly elevated ridge, which inclines a little 

 downwards as it extends forward, and dies away towards the anterior 

 end. The surface below this ridge is flattened, but very slightly 

 convex from above downwards, and, so far as preserved, is straight. 

 It shows a few deep narrow vascular grooves and markings for 

 vessels. The superior part of the side is obliquely twisted, be- 

 coming more and more horizontal behind, and more and more ver- 

 tical in front. In the middle of the side, both, in length and depth, 

 are four large foramina (fig. 10), placed close together in a line, 

 seven millimetres below the alveolar border, and 7 millimetres above 

 the longitudinal angle in the middle of the bone. These foramina 

 and their interspaces extend over a length of about 28 millimetres. 

 The anterior one descends vertically ; the three posterior ones enter 

 the bone obliquely, being directed downwards, forwards, and inwards. 

 From the hinder and inferior corner of the last foramen a slight 

 ridge is prolonged backwards, which makes an angle in the upper 

 margin of the jaw. The surface anterior to these foramina rounds 

 convexly from above downwards ; and below the third to sixth 

 alveoli there are about four minute foramina, and below the seventh 

 and eighth, only much nearer to the alveolar margin, two others. 

 It would thus appear as though a series of foramina had extended 

 along the bone, of which the middle four had become greatly de- 

 veloped. The thickness of the jaw from within outwards augments 

 along the line of the median lateral ridge ; in front it is about six 

 millimetres, in the middle 12 or 13 millimetres, and obviously in- 

 creases as it extends further backwards. What remains of the 

 inferior margin, the anterior 4 centimetres, is concave ; and the 



