A LOWER JAW OF LABTRINTHODON. 281 



ward prolongation of tho alveolar table, nearly an inch wide. From 

 this a sharp slightly elevated coronoid ridge [imperfectly preserved] 

 arises, which extends forward in the median line, originating at the 

 outer and anterior border of the articulation. Another ridge at right 

 angles to this, and somewhat thicker, is directed outward, so that a 

 concavity extends along the bone between them. This ridge extends 

 backward for 3 inches, inclined very slightly downward, and then 

 curves in a semicircle upward, to terminate just behind the articu- 

 lation. Below this ridge, under the articulation, is a shallow groove 

 less than j inch wide, and below this an elevated y-shaped 

 rudely sculptured mass of ornament, 1| inch long and J inch deep. 

 Below this is the broad v -shaped groove, with a few tear-like 

 ornaments. The hinder arm of the V extends upward behind the 

 articulation ; the front arm extends upward and forward ; both arms 

 are connected by the narrow curved groove below the articulation. 



Behind the V-shaped groove is a triangular piece of vascular 

 honeycomb-like ornament, 1J inch deep and lj inch long at its 

 base, where it is underlapped by the angular bone. It extends the 

 jaw externally behind the articulation. Nothing is behind the 

 straight inclined posterior line of this sculpture except the heel of 

 the bone — a subquadrate process half an inch deep, developed back- 

 ward and inward, so as with the angular bone below to nearly 

 enclose the hindmost termination of the articular bone, which is 

 greatly attenuated. The surangular bone is reflected behind the 

 articular element, so that superiorly it almost meets the splenial 

 element. Posteriorly, below the articulation, the bone is deeply 

 impressed on the irregular smooth hinder surface. 



The articular hone (figs. 1, 2, 3, e) forms a narrow strip on the inner 

 side of the jaw, oblique and curved backward, extending from the 

 articulation to below the heel of the surangular bone. The articular 

 surface is subrhomboid, extending further backward and outward on 

 the external margin, and further forward on the inner margin, which 

 slightly overhangs the bones beneath. It measures f inch from 

 front to back, and 1J from within outward. Its greatest oblique 

 measurement is less than 2 inches. The hinder margin (formed by 

 the surangular bone) rises above the anterior margin. From back 

 to front the area is semicylindrically concave, and marked with two 

 broad slightly elevated ridges with a longitudinal groove between 

 them, while a second incomplete sulcus forms the internal margin to 

 the articulation. 



Though the bones now described have a reptilian aspect, and 

 somewhat of a reptilian arrangement, they do not go far towards 

 suggesting the affinities of Labijriatliodon. There seems no reason 

 to doubt that the central hollow space in the jaw received the 

 primitive cartilage round which the bones were ossified ; and the 

 persistence of this character is rather a link with the lower than 

 with the higher Yertebrata. The jaw differs from the Batrachian 

 mandible in possessing well-developed angular and surangular ele- 

 ments, by which this cartilage and its articular ossification are en- 

 closed. But some reptiles, such as Crocodiles and the marine 



