!^as3a(iit>s».»«, ' 



SIO ON CEPHALASPIS AND PTERASPIS 



layer are directed nearly at right angles to those of the layers above 

 and below, so that under a low power the section appears cross-hatched 

 by a series of dark lines. The great vascular canals are well seen 

 traversing the successive lamella very obliquely. 



In flakes of the disk similarly treated, but containing more of the 

 middle and outer layers, fig. 2, it is obvious that the great canals divide 

 into the branches of the middle layer which have already been seen in 

 the vertical section, chiefly, if not only, along lines corresponding with 

 the apparent sutures between the so-called " polygonal scales." The 

 canals of the middle layer are very singularly arranged, passing from 

 their origin, across these sutural lines and nearly parallel with one 

 another, towards the centre of the adjacent "scales." The appearance 

 of distinct " scales,'' and of the curious lines along their boundaries, is 

 entirely due to this vascular distribution, the canals with their reddish 

 lining showing very distinctly against the whitish general substance. 

 In these views, again, the fissures by which the superficial layer is 

 interrupted in the sectional view are seen to be nothing more than the 

 ■expression of the valleys between the irregular and inconspicuous 

 tubercles into which the superficial layer is raised (PI. XIV. [XXXI.] 

 ■fig- 2). 



PTERASPIS. (PL XV. [XXXII.] ). 



A fragmentary specimen of Pteraspis Banksii (belonging to Mr. 

 Marston) affords by far the best view I have yet met with of the 

 general structure of the shield of this genus. A cast of the outer 

 surface is exhibited, and for the greater part of its extent the substance 

 ■of the shield is absent ; but in the centre a patch is left, exhibiting all 

 the layers in their natural condition and relations (fig. 2). 



The innermost layer (d') is composed of a reddish-white nacreous 

 substance, exhibiting a distinct appearance of lamination at its free 

 edges : its surface is somewhat uneven, and presents scattered rounded 

 apertures about :4^o-th of an inch in diameter. The edges of these 

 apertures were not unfrequently somewhat raised ; and their cavities 

 were full of a reddish matter. External to the innermost layer is the 

 middle layer id), composed of vertical plates of a laminated substance 

 of similar appearance to the inner layer, and varying in thickness from 

 TT^th of an inch downwards. These plates are so disposed as to 

 form a network, enclosing polygonal (4-5-6-sided) cells of an average 

 diameter of about -g-Vth of an inch. 



The inner apertures of these cells are closed by the inner layer. 

 P^xternally, they are also closed by a substance of the same nature as 

 their walls, but perforated by a variable number of apertures some- 



