THE TREMATASPID^. 



361 



oral plates, thus forming a regular series of openings leading into the interior 

 and increasing in size from behind forward. 



The oral plates form four or five transverse rows, the form of the smallest and 

 most anterior ones being imperfectly known. In the only known specimen that 

 has the plates in position, the large anterior pair were crushed and broken. 

 What appears to be one of the same plates has been found isolated and intact. 



Fig. 239. — Cross-section of the orbits of Tremataspis, showing the movable lateral eye stalks, the parietal eye, 

 the network of bony trabecials forming the floor of the median and lateral eye chambers, and the extension of the 

 dermal armor over the corneal surface of the lateral eyes. Semi-diagrammatic. X about 7 1/2. 



It has a small rounded notch at one end, and appears to represent the premaxil- 

 lary of Bothriolepis, although its form does not suggest a jaw plate. None of the 

 remaining plates resemble those of Bothriolepis. 



The exact location of the mouth is uncertain. It was probably in the ill-de- 

 fined triangular depression between the premaxillary plates. In any case it 

 must have been very small and narrow. 



The lateral eyes were on short stalks that could be raised above the level of the 



Fig. 240. — Bony plate from the neural surface of the basal joint of a cephalic appendage of Tremataspis. A , 

 External surface, showing a circular groove in the polished outer layer, made by striking the edge of the shield 

 in the swimming movements of the appendages; d, neck of the plate, attached by flexible membranes to one of the 

 openings on the haemal margin of the shield; B, inner surface of the plate. X , 10. 



shell, or lowered into the large spherical orbits, the floor of which consisted of a 

 basket work of bony trabeculae. The entire outer surface of the eye stalk and 

 the front of the eye itself was covered with a thin layer of the exoskeleton. (Figs. 

 238-239.) 



The marginal and post-orbital openings were originally filled with thin poly- 

 gonal plates; but the latter are usually absent, leaving shallow depressions with 

 scalloped margins, the floor consisting of the modified inner layer of the shell. 



