THE OLD RED SANDSTONE. 45 
which it seems to support. Such, at a first glance, is the 
appearance of the fossil. The body was of very considera- 
ble depth, perhaps little less deep proportionally from back to 
breast than the body of the tortoise; the under part was flat; 
the upper rose towards the centre into a roof-like ridge ; and 
both under and upper were covered with a strong armor of 
bony plates, which, resembling more the plates of the tortoise 
than those of the crustacean, received their accessions of 
growth at the edges or sutures. The plates on the under 
side are divided by two lines of suture, which run, the one 
longitudinally through the centre of the body, the other trans- 
versely, also through the centre; and they would cut one 
another at right angles, were there not a lozenge-shaped 
plate inserted at the point where they would otherwise meet. 
There are thus five plates on the lower or belly part of the 
animal. They are all thickly tubercled outside with wart-like 
prominences, (see Plate I., fig. 4; ) the inner present appear- 
ances indicative of a bony structure. The plates on the 
upper side are more numerous and more difficult to describe, 
just as it would be difficult to describe the forms of the vari- 
ous stones which compose the ribbed and pointed roof of a 
Gothic cathedral, the arched ridge or hump of the back re- 
quiring, in a somewhat similar way, a peculiar form and 
arrangement of plates. The apex of the ridge is covered 
by a strong hexago.aal plate, fitted upon it like a cap or hel- 
met, and which nearly corresponds in place to the flat central 
plate of the under side. There runs around it a border of 
variously formed plates, that diminish in size and increase in 
number towards the head, and which are separated, like the 
pieces of a dissected map, by deep sutures. They all pre- 
sent the tubercled surface. The eyes are placed in front, on 
a prominence censiderably lower than the roof-like ridge of 
