1858.] HUXLEY-— CEPHALASPIS AND PTERASPIS. 27^ 



work, the canals of which appear indeed to open into the bottom of 

 the interspaces. 



A structure in every essential respect similar to that just described 

 is to be found in all other completely ossified parts of the cephal J c 

 shield, whether dorsal or ventral. In other regions of the dorsal 

 part, however, the lamination of the inner layer is far more marke d ; 

 and as a general rule the middle layer in these parts of the shield is 

 thinner and contains fewer layers of lateral vascular ramuscules. The 

 like is true of the inner part of the ventral region, in which only 

 a single layer of close-set vascular canals makes its appearance 

 (PI. XIV. fig. 5). The flexible part of the ventral layer appears to 

 be composed of the lamellar inner layer only ; and the thick margins 

 of the disk resemble the spine in structure. 



The structure of the ventral layer, enclosed as it is on both sides by 

 the matrix, is usually very well displayed in sections, and the b etter, 

 on account of the dark reddish-brown hue which is acquired b y the 

 matrix, for some little distance from its line of contact* with the ani- 

 mal substance. But neither in these nor in any other secti ons can 

 any trace of bony substance be discovered beyond that whic h enters 

 into the composition of the thin cephalic shield itself. I believe, 

 therefore, that the so-called " fibrous bone" is nothing but the sur- 

 face of the matrix impressed by the inner surface of the disk, and 

 stained of a darker colour than elsewhere. 



If flakes of the inner layer of the shield be detached and well soaked 

 in hot Canada balsam, they become transparent, and their structure is 

 well displayed in a superficial view (fig. 3). At their broken edges, the 

 lamellae of which they are composed are seen cropping out one beyond 

 the other ; but their most striking feature consists in the long lines 

 of lacunae which lie in parallel and equidistant series in each layer, 

 so that under a low power it appears to be composed of broad flat 

 fibres arranged side by side. The axes of the lacunae of each 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 lamellae 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 th e 

 expression of the valleys between the irregular and inconspicuou s 

 tubercles into which the superficial layer is raised (PI. XIV. fig. 2). 



