380 Proceedings of the Royal Physical Society. 
in Pl. XL, Fig. 3 (Brit. Mus. P. 8297), the tubercles 
are much more completely preserved than is usually the 
case. 
This fragment measures 24 inches in length by 12 inch in 
breadth ; its greatest thickness is } inch, but this diminishes 
to =; at the middle of the side, which in the figure is placed 
to the left. On the external surface there is, along the side 
alluded to, a space bare of tubercles, and about half an inch 
in breadth; the rest of the outside of the plate is covered 
with tubercles of the dimensions stated, but the different 
sizes are not indiscriminately mixed, the smaller ones pre- 
vailing on the side opposite to the non-tuberculated area. 
These tubercles are in this instance pretty close together, 
but far enough apart to show between them some of the 
vascular surface of the middle layer below. Each of the 
tubercles, if perfect, shows when magnified (Fig. 4) an 
apical boss or cap of ganoine, which may often be seen to 
be nicked or crenulated round its margin with a variable 
number of identations ; but this ganoid cap does not reach to 
the base of the tubercle, as may be seen in the figure. On 
the specimen figured, two wavy grooves are also seen on the 
surface. . 
In other examples the tubercles are more distantly 
scattered, so that there may be considerable intervals 
between them, and in worn specimens they are usually 
broken through and decapitated, or rubbed down. 
The under surface shows a tissue of a dense laminated 
character, as in other species of the genus. 
I have included these fragmentary plates under Psam- 
mosteus, on account of the similarity of the vascular network 
of the middle layer to that in the other species of the genus, 
although the nature of the tuberculation is somewhat 
aberrant. Nevertheless, even this external ornament cor- 
responds more to that of Psammosteus than of any other 
plates with which I am acquainted. 
