SI2 ON CEPHALASPIS AND PTERASPIS 



TTuth of an inch by the second layer, ^iyth of an inch by the next, 

 and ir-j^xjth by the outermost layer. 



The outer layer (a) appears to consist of a series of papillary eleva- 

 tions which have a broad free end, and are attached by narrow bases, 

 so that a triangular interspace with its apex outwards is left between 

 every pair of elevations. The matrix filling these interspaces, and for 

 some distance in the immediate vicinity of the outer surface, is much 

 darker than elsewhere, and has a deep brown hue. The attached ends 

 of the elevations pass into a whitish substance, which, under this 

 power, looks similar to their own. It is traversed by many reddish 

 canals, which send diverticula into the elevations (d) ; and hence this 

 substance clearly represents the " reticular layer " of P(. Banksii. At 

 intervals of about yL-th to ilo-th of an inch or thereabouts, thin septi- 

 form processes are given off from the reticular layer, and pass perpen- 

 dicularly inwards to the inner layer ; they thus subdivide the second 

 layer into a series of irregularly quadrate spaces, corresponding with 

 the prisms seen in the superficial view. 



The inner layer is, like the rest, whitish, and is traversed parallel 

 with its surface by four or five much whiter streaks, so that it appears 

 to be composed of only a corresponding number of lamellae ; but on 

 allowing the light to pass through the section, it is at once obvious 

 that each of these apparent lamellae is in reality made up of many of" 

 the primitive laminae which constitute the inner layer, and that the 

 bright and dull white streaks are due entirely to a difference of texture 

 or composition in the successive groups of laminae. 



Under a high power the laminae are seen to have a thickness of 

 about j-jVirth of an inch, and to run nearly parallel with, and closely 

 applied to, one another. They present an indistinct vertical striation,, 

 but exhibit no canals nor lacunae. The septa of the second layer are 

 composed of similar laminae, but less distinct, and curved in various 

 directions, usually more or less parallel to the walls of the large cavities 

 which they bound. A fragment of the inner layer (fig. 4), rendered 

 transparent by Canada balsam, and viewed by transmitted light, 

 shows that it contains no lacunae ; nor have I been able to detect any 

 distinct structure in its laminae, unless an obscure and very delicate 

 striation, visible here and there, may be regarded as such. 



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 



