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



Pteraspis. (PL XV.) 



A fragmentary specimen of Pteraspis Banksii (belonging to Mr. 

 Marston) affords by far the best view I have yet met with of the 

 general structure of the shield of this genus. A cast of the outer 

 surface is exhibited, and for the greater part of its extent the sub- 

 stance of the shield is absent ; but in the centre a patch is left, ex- 

 hibiting all the layers in their natural condition and relations (fig. 2). 



The innermost layer (d) is composed of a reddish-white nacreous 

 substance, exhibiting a distinct appearance of lamination at its free 

 edges : its surface is somewhat uneven, and presents scattered rounded 

 apertures about yjjxfth of an inch in diameter. The edges of these 

 apertures were not unfrequently somewhat raised ; and their cavities 

 were full of a reddish matter. External to the innermost layer is the 

 middle layer (c), composed of vertical plates of a laminated substance 

 of similar appearance to the inner layer, and varying in thickness from 

 ^-jLjth of an inch downwards. These plates are so disposed as to 

 form a network, enclosing polygonal (4-5-6-sided) cells of an average 

 diameter of about ^y-th of an inch. 



The inner apertures of these cells are closed by the inner layer. 

 Externally, they are also closed by a substance of the same nature 

 as their walls, but perforated by a variable number of apertures some- 

 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). 



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

 by another of Mr. Marston' s specimens of Pt. Banksii. This ex- 

 hibits, 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 

 outwards and their bases juxtaposed. The summits of the ridges are 

 as much as y^th to yy^th 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 appearances 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- 



