ON CEPHALASPIS AND PTERASPIS SOp. 



before referred to. Hence, when the section is viewed by transmitted 

 light, the vascular canals are very distinct, and appear to end abruptly 

 in the deep half of the superficial layer, while the tubuli have the 

 aspect of fine, clear, sparsely ramified lines, by no means so readily 

 visible. In some cases they seem to open on • the surface. This 

 substance, it will be observed, corresponds very closely in structure 

 with the " cosmine " of Professor Williamson. I have been unable to 

 find any trace of a " ganoin " layer external to it. 



The superficial layer does not form a continuous whole, but is seen 

 in the section to be divided into masses of various length by inter- 

 spaces or gaps, which extend as far as the superficial vascular net- 

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

 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 cephalic 

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

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

 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. [XXXI.] 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 better, 

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

 matrix, for some little distance from its line of contact with the animal 

 substance. But neither in these nor in any other sections can any 

 trace of bony substance be discovered beyond that which enters into 

 the composition of the thin cephalic shield itself I believe, therefore,, 

 that the so-called " fibrous bone " is nothing but the surface 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 lacuna: 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 lacuna; of each 



