294 



THE DERMAL SKELETON. 



connect with the layer above and below. The diameter of the canals in the inner 

 layer is the largest and the main canals of this layer run at right angles to the sur- 

 face ridges. (Fig. 196, B.) The diameter of the canals in the several layers dimin- 

 ishes toward the outer surface; at the same time the apparent trend of the main 

 canals of a layer gradually shifts, so that in the outermost one the main horizontal 

 canals run parallel with the surface ridges, one canal running lengthwise along the 

 basal portion of each ridge. In sections parallel to the outer surface as well as in 

 cross-section, it is readily seen that each ridge canal opens right and left into the 

 bottom of the grooves, s.c., between the ridges, and at pretty regular intervals sends 

 a short loop upward into the ridge itself, c.ca. From the summit of these ridge- 



A 



FIG. 197. 



FIG. 198. 



FIG. 197. Cross-section of a small portion of a shield especially well preserved, and probably belonging to 

 Pteraspis; highly magnified, showing the axial core and the sharply laminated structure of the trabeculae. 

 FIG. 198. Trabecuke of Ateleaspis. 



loops arise the radiating dentinal canals of the ridges. These terminal canals 

 resemble those of Tremataspis, but they do not contain any lacuna-like dilatations. 

 4. The layer of surface ridges and the intervening grooves form the fourth or 

 outer layer of the shield. 



The substance of the shell consists of a series of plates and trabeculae; the 

 cut surfaces of the latter present a very distinct concentric lamination that is 

 precisely like the laminated trabeculae in Limulus. In each plate or bar there is 

 an axial core of a darker, yellowish-brown color; it is also distinguished by a 

 change in the distinctness of the lamination. The outermost laminae are crossed 

 by innumerable fine lines that produce the appearance shown in Fig. 197. In 

 other specimens that have been preserved in a little different manner, some 

 of the canals are filled with air so that they become very distinct, like the pore 

 canals of Limulus, or the air-filled canaliculi of typical bone cells. 

 These canaliculi begin at the surface of the trabeculi and extend at right angles to 

 the lamellae into the axial core. There they bend nearly at right angles and some 

 of them terminate in slender, spindle-shaped, or elongated dilatations, the long 

 axis of which generally lies parallel with the long axis of the core. These terminal 

 dilations or primitive lacunae are readily seen with a magnification of about 600 



