1858.] HUXLEY CEPHALASPIS AND PTERASPIS. 275 



ing to the Geological Society. The inner layer is thin, whitish, and 

 nacreous, and presents, scattered over its surface, apertures of a similar 

 character and size to those shown by Pt. Banksii. 



The next layer appears, at first, to be very different, inasmuch as 

 it seems to be composed of irregular reddish prisms with white inter- 

 spaces. The prisms have a diameter of ^j-th of an inch, more or less. 



The reticular layer is hardly distinguishable in this view ; but when 

 the apparently prismatic substance is broken away, either a thin 

 filmy outer substance is visible, or a peculiar striation. A thin sec- 

 tion of the shield ofPt.Lloydii (fig. 1 ), taken perpendicularly both to 

 its plane and to its long axis, exhibits the following appearances when 

 viewed with a low power by reflected light.' 



The total thickness of the section is about ^ t n °* an iuch, and of 

 this amount about -j-nyth of an inch is occupied by the inner layer, 

 y^th of an inch by the second layer, ^-jj-^th of an inch by the next, 

 and yj- th 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 (b) ; and hence this 

 substance clearly represents the " reticular layer" of Pt. Banksii. At 

 intervals of about y^th to yro tn 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 4; ^ 00 th 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 di- 

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

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



