272 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



in diameter, whose inner openings correspond with the brown spots 

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

 their course towards the middle layer (fig. 1, 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 anastomose 

 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 lacunae, and by the indistinctness of the 

 lamination as compared with that of the deep layer. Such stria- 

 tums 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 there- 

 fore greatly inclined to the course of the great canals), and anasto- 

 mose 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 dia- 

 meter of about -gxnyth 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) 

 sometimes 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 

 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- 



