DERMAL SKELETON OF PTERASPIS. 



293 



Pteraspis. 



Of the genus Pteraspis only a part of the cephahc armor and a few scale-Hke 

 structures belonging to the anterior part of the trunk are known. 



The boat-shaped dorsal shield (Fig. 245), is composed of seven portions, 

 marked off on the outer surface of the shield by furrows, and on the inner surface 

 by ridges. In young specimens the rostrum and the central disc may be found 

 separately. In each piece the ornamental surface ridges and furrows, which look 

 much like the wavy lines on the finger tips, are arranged in concentric lines parallel 

 with its margins. This fact, together with other considerations, led Lankester to 



Fig. 196. — A, Cross-section of the shield of Pteraspis, at right angles to the surface ridges; B, Tangential section 

 through the outer layers, and nearly parallel with the outer surface. 



believe that each piece ossified from a separate center, and that their complete 

 anchylosis occurred only in the adult. 



Along the lateral margins of the shield, in the rostrum, and near the posterior 

 dorsal spine, the shell is greatly thickened, and consists of a network of bony tra- 

 beculae with irregular spaces between. In the median portions it is of a more 

 uniform thickness, about 0.6 to 0.8 mm. 



In sections across the ridges (Fig. 196, A), the shell is seen to consist of four 

 principal layers: i. A thick inner wall, h.l., composed of many parallel layers of 

 uniform thickness, perforated here and there by openings that lead from the interior 

 of the head into the overlying cancellous spaces. 2. A cancellated layer, f.?., consist- 

 ing of polygonal chambers, c, separated by thin vertical walls that are perforated 

 here and there by narrow lateral passages. 3. Three or four layers of canals, 

 V.I., each layer forming a close four-sided meshwork from which vertical canals 



