ON CEPHALASPIS AND PTERASPIS 51I 



what smaller than those in the inner layer {b). The inner surface of 

 this substance presents in many cases a striation more or less parallel 

 to the sides of these apertures ; and when it is broken away the 

 thickness of the layer which closes the outer apertures of the cells is 

 seen to be permeated by numerous small canals which give it a sort 

 of worm-eaten or reticulated appearance. I will call this the " reticular 

 layer." Lastly, outside the reticular layer is a white substance, very 

 imperfectly visible in this specimen, in which no canals are visible, and 

 which constitutes the external layer («). 



A view, the precise complement of that just described, is afforded 

 by another of Mr. Marston's specimens of Pt. Banksii. This exhibits, 

 for the most part, a cast of the internal surface ; but towards the edge 

 a considerable portion of the shield is left in a very perfect state of 

 preservation, and with its external surface intact. The external layer 

 is produced into strong ridges, the summits of which are turned out- 

 wards and their bases juxtaposed. The sumrrits of the ridges are as 

 much as xl^j-th to x4-(jth of an inch apart In some cases they were 

 sharply angular, in others more rounded. Where this layer was broken 

 away, the reticular layer beneath it, and the polygonal cells of the 

 next layer were well displayed. The bottoms of these cells were seen 

 to be closed by the inner layer, and in this apertures were visible, 

 corresponding with those on its inner surface. I have not examined 

 transverse sections of this species ; but the structure of Pt. Lloydii is 

 so similar, that its transverse section perfectly elucidates the appear- 

 ances presented by P. Banksii. 



I have seen no specimen exhibiting the unaltered external surface 

 of Pt. Lloydii ; but its internal surface and its other layers, where the 

 inner one is broken away, are well displayed in two specimens belong- 

 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 ^V* 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 section 

 of the shield of Pt. Lloydii (fig. i), 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 ^Vth of an inch, and of 

 •this amount about -^l-^'ih. of an inch is occupied by the inner layer. 



