508 ON CEPHALASPIS AND PTERASPIS 



almost black when viewed by transmitted light, in consequence of the 

 quantity of air retained in the multitudinous lacunae and canaliculi. 



Large vascular canals, measuring from -z-J-Q-th to xTirth of an inch 

 in diameter, whose inner opening corresponds with the brown spots 

 on the inner surface, traverse the innermost layer very obliquely, in 

 their course towards the middle layer (fig. i, e). Their branches are 

 few, and for the most part run parallel with the main trunk ; but they 

 give off a great multitude of minute canaliculi, which an astomose with 

 those of the nearest lacunae. Such of these canals as I have seen in 

 section were oval, their long diameters being parallel with the planes 

 of the lamellae. In the specimen described the walls of the canals are 

 lined with a reddish matter (like oxide of iron) ; and a similar 

 substance obstructs many of the canaliculi. 



The middle layer (c) is distinguished from the inner by the rarity 

 ■or entire absence of the lacuna, and by the indistinctness of the 

 lamination as compared with that of the deep layer. Such striations 

 of the nearly homogeneous base as seem to indicate lamination are, 

 in the middle and inner parts of the middle layer, so disposed as to be 

 nearly perpendicular to those of the deep layer, appearing to follow 

 the course of the vascular canals. 



The latter are continuous with the large vascular canals of the 

 deep layer, but they are smaller and form a close network. Each of 

 the large canals, on reaching the middle layer gives off several 

 branches, which run nearly parallel with the surface (and therefore 

 greatly inclined to the course of the great canals), and anastomose 

 with those around, above, and below them. In this particular part of 

 the disk, in fact, a large canal gives off as many as three tiers of these 

 lateral branches, separated from one another by not much more than 

 their own diameter, and all ramifying and anastomosing with one 

 another. These lateral vascular canals have at first a diameter of 

 about -g-^o-th of an inch ; but many of their anastomotic branches are 

 much smaller. 



Sooner or later all these branches appear to end in a close " super- 

 ficial network," b, which lies in the boundary between the middle and 

 the superficial layers. The latter or third layer of the disk {a) some- 

 times appears structureless, at others presents an obscure vertical 

 striation, as if it were, like enamel, made up of minute fibres. The 

 superficial vascular network sends into it a great number of minute 

 short processes, which branch out abruptly at their ends, like a thorn- 

 bush or a standard rose-tree, and end in excessively fine tubuli, like 

 those of dentine. The tubuli appear empty and are much finer than 

 the vascular processes, which are usually full of the dark red matter 



