374 THE OSTRACODERMS. 



laterally and inward on the visceral side of the oral membrane, serving, no doubt, 

 for the attachment of muscles, s.p. 



The visceral aspect of the premaxillae is nearly smooth, except for a very 

 prominent, transverse, sharp-edged ridge, which evidently served for the attach- 

 ment of muscles that moved them in a median or lateral direction. (Fig. 254, C.,ir.) 



The premaxillae moved independently to and from the median line, bringing 

 their stout crushing or biting edges together. They are sometimes found in a 

 nearly vertical position, or even thrown forward in front of the head, with their 

 ventral surfaces facing dorsally. Thus it is probable that, like two great lids or 

 covers, they could swing forward and backward on the muscles and the mem- 

 brane attached to their anterior visceral surface; it is improbable that normally 

 they ever passed beyond the vertical position during life. (Fig. 251.) 



Fig. 255. — The ocular and olfactory plates of Bothriolepis, enlarged. A part of the rostral and lateral plates have 

 been removed on the left, in order to expose the deeper lying sclerotics and the inner end of the ethmoid. 



The mandibles are peculiar S-shaped plates lying behind, or more frequently 

 underneath, or dorsal to, the premaxillae. (Figs. 248, 251, 254, md.) The 

 median end of each mandible has a smooth, rounded, anterior, or ventral edge, 

 and a finely ornamented, convex outer surface. Its visceral surface is deeply 

 concave. The lateral arm of the mandible is narrow and smooth and lies inside 

 the oral membrane. 



The mandibles are usually widely separated in the median hne, each being 

 quite independent of the other; they are held in place by the tough skin in which 

 their median ends are imbedded. They appear to have had a free rotary move- 

 ment, their ventral edges swinging forward and backward; at the same time, 

 their median ends could be drawn together and thrown forward. 



The difference in position and structure between the mandibles and pre- 

 maxilte makes it improbable that one pair acted directly against the other. The 

 mandibles probably pushed the food forward and inward, where it could be 

 crushed or cut between the stout margins of the premaxillae, after the manner 

 that prevails among the arthropods. 



