360 



THE OSTRACODERMS. 



caliber, and terminate in coarse, dentinal canals that dilate at intervals, forming , 

 in the outer shell layer vertical rows of multipolar lacunae; invs^ardly they lead into 

 the cancellae, and hence via coarse vertical canals, through the basal layer into 

 the head. The second set are uniform in diameter and open outwardly by clear- 

 cut conical chimneys. (Fig. 205.) 



It 

 V/ 



Fig. 237.- 



-Restoration of Tremataspis, seen from the haemal surface, and showing the arrangement of the circum- 

 oral plates, X about r 1/2. 



Oral Region. — There is a large opening on the anterior haemal part of the 

 head, near the center of which lies the mouth. (Fig. 237.) It is surrounded by 

 close-fitting plates, seldom preserved in place. The opening in which these 

 plates belong is bounded in front and on the sides by the narrow, overturned rim of 

 the neural shield. On the anterior rim is a triangular, ill defined plate, adhering 



Fig. 238. — Photograph of the orbital region of the shield of Tremataspis. X about 3. 



to the inner surface of the neural shield. It has a median, oval ridge b, with a 

 roughened or porous texture. On either side is a polished, conical projection 

 of the narrow rim, a. On the lateral and posterior margins of the opening are 

 eight or nine pairs of thick-lipped, semi-circular incisions, whose concave surface 

 is conspicuously porous. There are corresponding incisions on the adjacent 



