50 THE OLD RED SANDSTONE. 
Sil. Syst.,Plate VII.) The upper part, or head, forms a cres« 
cent; the body rises out of the concave with a sweep some- 
what resembling that of a Gothic arch; the outline of the 
whole approximates to that of an egg, the smaller end termi- 
nating in a sharp point. Let him remark, further, that this 
creature was a crustaceous animal, of the crab or lobster 
class, and then turn up the brief description of the Old Red 
Sandstone in the same volume, page 454, and mark the form 
of the Cephalaspis, or buckler-head—a fish of a formation 
immediately over that in which the remains of the trilobite 
most abound. He will find that the fish and the crustacean 
are wonderfully alike. The fish is more elongated, but both 
possess the crescent-shaped head, and both the angular and 
apparently jointed body.* They illustrate admirably how 
two distinct orders may meet. They exhibit the points, if 1 
may so speak, at which the plated fish is linked to the shelled 
crustacean. Now, the Coccosteus is a stage further on; it is 
more unequivocally a fish. It is a Cephalaspis with an artic- 
ulated tail attached to the angular body, and the horns of the 
crescent-shaped head cut off. 
Some of the specimens which exhibit this creature are 
* Really jointed in the case of the trilobite; only apparently so in 
that of the Cephalaspis. The body of the trilobite, like thet of the 
lobster, was barred by transverse, oblong, overlapping plates, and be- 
tween every two plates there was a joint; the body of the Cephalaspis, 
in like manner, was barred by transverse, oblong, overlapping scales, 
between which there existed no such joints. It is interesting to ob- 
serve how nature, in thus bringing two such different classes as fishes 
and crustacea together, gives to the higher animal a sort of pictorial 
resemblance to the lower, in parts where the construction could not 
be identical without interfering with the grand distinctions of the 
classes. 
