TREMATASPIS. 



291 



but much fainter. The surface is dotted with minute circular openings arranged 

 in irregular rows that usually correspond with the meshes of the underlying canals. 

 The shell is about 0.3 mm. thick. The inner portion consists of horizontal 

 lamellae of uniform width, interrupted by large irregular chambers. (Fig. 193.) 

 The lamellae appear to have consisted originally of fibrous strands arranged with 



FIG. 193. Cross section of the exoskeleton of Tremataspis. 



great regularity into parallel bundles, those in adjacent layers running at right 

 angles to one another. Viewed from the inner surface of the shell the strands of 

 the lamellae look like the warp and woof of a coarse cloth (Fig. 194). Between 

 the layers are flattened lacunae, sometimes filled with a reddish-brown substance 

 that gives them a very striking resemblance to living pigment cells. Canaliculi 



,.,-exp. 



tr. 



FIG. 194. Exoskeleton of Tremataspis, seen from the inner surface. Enough of the outer surface has been removed 



to make the remainder semi-transparent. 



radiate from the lacunae and anastomose with those in the layers of bone above 

 and below. 



The substance of the shell is penetrated by two sets of canals. The deeper 

 set forms a horizontal meshwork of uniform caliber, opening to the outer surface 

 at frequent intervals by short conical chimneys. (Fig. 193, sn.c.) This system 

 of canals is generally filled with a peculiar matrix that is either colorless and similar 



