466 H. G. SEELET ON A PURBBCK ORXIIIIOS-UKIAX. 



This jaw is naturally compressed from above downward, so as to 

 be scarcely more than an J inch thick. The specimen gives no con- 

 clusive evidence that the jaw terminated anteriorly with the fragment 

 of bone preserved, since the upper part of the bone of the anterior 

 three inches of the symphysis is broken away and only the thin, 

 inferior, investing, external layer of bone remains, and of that the 

 anterior outline is not entire. This bone has its inner surface 

 rough, a structure which probably indicates a roughened external 

 vascular condition. The underside of the jaw is flattened, and very 

 slightly convex from side to side. The anterior terminal inch is 

 bent up slightly towards the upper jaw ; and this probably shows 

 that nearly the whole symphysis is preserved. The upper and lower 

 surfaces of the symphysial portion of the jaw converge laterally to 

 meet in the line of the teeth : the upper palatal surface has a flat- 

 tened aspect, but is gently convex from side to side. 



The sockets for the teeth appear to have been scarcely more than 

 | of an inch deep, and to have extended along at least S inches of 

 the jaw. They are so arranged that in front of the jaw they may 

 have been directed outward horizontally, while in passing backward 

 they become steadily less and less inclined, till the hindmost sockets 

 appear to have been vertical. A similar arrangement is found in 

 some species of Plesiosaurus. and more or less in the buccal margin of 

 the jaws of birds and many mammals. Each socket is ovate, with 

 the anterior end a little raised : it is margined by an elevated rim, 

 which is more prominent on the under, or outer, than on the upper 

 or inner margin of the socket. In the middle of the jaw tbere 

 appear to be 7 teeth in the space of an inch. The teeth appear to 

 have extended for at least 3| inches behind the symphysis : but each 

 ramns narrows rapidly from side to side to about 4 inch in width, 

 becomes deeper, and the palatal surface becomes rather more convex 

 from side to side. The rami appear to widen again behind the teeth; 

 but this is not quite clear, since for the hinder 5 or 6 inches only the 

 lower surface of the fractured bone is preserved. 



Sometimes the teeth are so closely packed together as to be parted 

 only by a paper thickness, while sometimes interspaces occur as wide 

 as a tooth. The palatal surface of the jaw is striated longitudinally. 

 The palatal groove is so deep as to extend more than halfway 

 through the jaw : it widens where it terminates behind to ^v inch : 

 and its basal surface is there elevated so as to divide the groove and 

 prolong each half of it for some distance beyond the symphysis, 

 down the inferior and inner side of each ramus. 



There is no evidence to show what elements of the lower jaw are 

 preserved, or to indicate the length of bone lost between the hind- 

 most fracture and the articulation with the quadrate bone. On the 

 right inner side a small smooth surface of bone is shown ; and this is 

 probably the suture between the dentary and the angular bones. 



The vertebra. 



This specimen demonstrates the external layer of bone to have 

 boon more than usually thin, with extremely delicate cancellous 



