442 
POPULAR SCIENCE REVIEW. 
stones in Scotland, and underlie tlie fishbearing strata, a few 
specimens are obtained of one species of the three genera we 
are about to describe ; but they are far from being so abundant 
as they are in the Corns tones 33 of Herefordshire. So much 
for the strata of England and Scotland — the fish which belong 
to the Lower Old Red Sandstone of England in which we may 
include the “ passage beds 33 spoken of above, are of three 
genera, the species being not yet satisfactorily determined. 
These are Cephalaspis, Anchenaspis, Pteraspis. It must be 
understood that no species of these genera have yet been 
found in the higher beds of Scotland, nor have any of the 
species of the higher Scotch beds been found in the lower 
English beds — the upper and lower groups are quite distinct. 
The three genera, Cephalaspis, Anchenaspis, and Pteraspis, 
together form a family called the Cephalaspida3, since they all 
bear some resemblance in external form to one another. The 
Cephalaspis is the best known amongst these fish, and was 
the first discovered. Agassiz gave it this name on account of 
the peculiar form of the head, which he compared to a shield 
or buckler (fce^aAr}, head ; shield) ; whilst at the same 
time he associated with the true Cephalaspis another fish which 
has since been shown to be quite distinct, and has been placed 
in another genus, Pteraspis. The original type of the genus 
was the Cephalaspis Lyellii, which is by far the commonest 
species ; and at the time it was described, many specimens of 
the head were known from Herefordshire, and some beauti- 
fully preserved examples in which the body was attached; 
one of these last was from Scotland. In our plate (fig. 1) the 
head of the G. Lyellii is drawn ; the various details have been 
taken from two or three specimens, since no single specimen 
can be obtained perfect enough to present all that is known of 
its form. At first sight one would be inclined to regard this 
queer shovel-like body as anything but the head of a fish ; 
and, indeed, for some time it was quite excusably considered 
to be the head-plate of some crustacean allied to the Trilobites 
or King-crab. The resemblance it bears to the cephalic 
shield of the Limulus is very remarkable. It is a semi- 
circular raised disc, the angles of which on either side are 
terminated by two long processes or horns which are very 
finely serrated ; paleontologists call these the “ lateral cornua. 
In the median line the disc is produced backwards, and rather 
upwards, into a sharp spine which is called the “ posterior 
cornu.” The orbits are placed close together, very nearly in 
the centre of the disc. There are other irregularities on the 
surface, which correspond probably to the disposition of the 
soft parts which were placed beneath this shell. No lower 
jaw, teeth, mouth, or any organ of the sort have yet been 
