THE ANTIARCHA. 



373 



The pre-oral chamber lies on the anterior ventral surface; its rim is formed 

 by the anterior margins of the dorsal and ventral shields, which were apparently 

 fringed with short fleshy papillae. Across the roof of the opening there is a tough 

 circum-oral membrane in which are imbedded the large premaxillary plates, 

 the mandibles, and the two narrow bony bands that covered the hyoids. The 

 membrane extends backward, underneath the anterior ventro-laterals, as far as 

 the transverse ridge on their inner surface, to which it appears to be attached 

 (Fig. 251, 254, om.) 



p.s 



FIG. 254. E, The oral region of Bothriolepis. On the right, the anterior ventro-lateral plates have been re- 

 moved exposing the dermal armor of the double hyoid arch, imbedded in the circumoral membrane. The latter is 

 continuous with the ridge, b, on the inner surface of the anterior ventro-laterals. A , The mandibles of Bothriolepis, 

 with the smooth, toothless margin rotated outwards; B, same, rotated inwards; C, longitudinal, vertical section, 

 a little to one side of the median line, showing the pre-maxillary and mandibular plates in their normal positions; 

 D, cross-section of the premaxillae in the oral region, showing the thickened, crushing, or cutting edges of the 

 premaxillary plates. 



The premaxilla are thin, concave plates of dermal bone continuous with the 

 oral membrane on the sides and in front, but with free median and posterior 

 margins. (Fig. 254.) The exposed surface presents the characteristic surface 

 ornament, together with a sharply bent, sensory groove. The rounded posterior 

 margin is smooth and bevelled, ending in an extremely thin edge, nearly the 

 whole length of which is broken into minute irregular tooth-like serrations. The 

 median margin is very thick, with sharpened edges which in the older individuals 

 become rounded or smooth through use. The anterior margin has a prominent 

 shoulder; it is uniformly thin and rounded, and is continuous with the oral mem- 

 brane. The lateral margin is narrow and slightly concave; a broad spur extends 



