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



A similar disposition of curved laminae can be traced in the " re- 

 ticular layer ;" but in the elevations of the external layer, such 

 laminae are no longer distinctly visible, although here and there traces 

 of them may be seen. Each elevation, in fact, nearly resembles the 

 tooth or dermal defence of a placoid fish. It contains a central 

 cavity, commonly filled with a dark red matter, which usually occupies 

 the centre of the basal half of the elevation and then suddenly 

 ends in a number of excessively minute branches, which pass towards 

 the surface, ramifying as they go, and closely resembling the canals 

 of dentine or cosmine. They appear to terminate on the surface, on 

 which I have been unable to discover any trace of laminated struc- 

 tureless ganoin. The central canals of the elevations open internally 

 into the network of vascular canals which lies in the reticular layer. 

 These canals rarely exceed y^-th to -g^njth of an inch in diameter, and 

 they are rendered particularly obvious by the dark red granules with 

 which their walls are dotted. 



Internally they open directly into the interspaces of the septa which 

 connect the reticular with the inner layer, and the granules are con- 

 tinued on to the walls of the septa, which are themselves occasionally 

 traversed by short canals. The interspaces (e) are full of a more 

 or less transparent inorganic matter, identical with that of the matrix. 

 It follows, therefore, that the "bony prisms" or "granules" which 

 have been described have no existence, these so-called prisms being 

 nothing but the matrix which has filled up the cavities of the poly- 

 gonal cells, visible in their natural empty condition in Pt. Banksii. 

 Canals resembling those of the reticular layer, as I have said, traverse 

 some of the septa and put their chambers in communication. 



In the section under description, the inner layer is for the most 

 part devoid of canals ; but one (/) is exhibited very beautifully. It has 

 in the middle a diameter of about -^-^th of an inch, but is wider at 

 both ends, and traverses the inner layer almost perpendicularly. The 

 laminae are bent outwards for a certain distance, where they impinge 

 upon its walls. 



The structure just described is that of the central part of the section. 

 At one of its ends, near the margin of the disk, the arrangement 

 of the vascular channels is more like that in Cephalaspis, — the 

 reticular layer assuming a much greater development, and the 

 areolar character of the sinuses of the second layer becoming greatly 

 obscured. 



On comparing together the appearance of a section with those pre- 

 sented by the internal and external views of Pteraspis, there can be 

 no doubt that the elevations of the outer layer of the one are the 

 sections of the ridges of the other ; and it is remarkable that there 

 should be so striking a difference in the form of these ridges in Pt. 

 Banksii and Pt. Lloydii. The ridges seen in concave casts probably 

 always correspond with the whole interspaces between the ridges of 

 the outer layer in Pt. Banksii ; but it is quite conceivable that in 

 Pt. Lloydii the ridges, in consequence of their peculiar form, might 

 sometimes be held by the matrix and sometimes not ; so that at one 

 time the ridges of the cast would be very narrow, corresponding only 



