406 
THE GEOLOGIST. 
a guess. "W'e regard the whole as forming an abdominal plate, com- 
posed of dilfereut pieces, joined by sutures, and which covered the un- 
der side of the head and, it may be, part of the body of the Pteraspis. 
The pieces we possess are numbered in the diagram 1, 2, 3, and the 
others have not occurred to us. 
In 1 we cannot discover any 
trace of the eye-socket, which, 
had this been the anterior por- 
tion of a dorsal plate, ought to 
have been displayed ; but in 2 
there is a round orifice, to which 
a sucking apparatus might have 
been well attached. If this in- 
terpretation be correct, and be 
confirmed by further evidence, 
then, at length, we have reached 
the method in which the Pter- 
aspis and its kindred Cephalas- 
pis sought and received their 
food in the waters. The sepa- 
rate plates seem to be bone, 
composed like that of the ce- 
phalic buckler, and were ap- 
parently joined together by 
deep sutures. The bone cover- 
ing the upper surface of the 
head presented a solid mass to 
any opposing object ; but that 
covering the under surface, as 
less exposed, was formed of dif- 
ferent pieces, and thus flexible 
wherewithal. And we have ob- 
served that Plate 1 covers Plate 
2 by a deep marginal socket ; 
so that the plate to which a 
■^^S- 3. sucker might be attached could 
not be torn from its place without the resistance of the other. 
COEEESPO^sDENCE. 
Origin of Flint Veins in ChalTc. 
Deak Sie, — A short time since a paper appeared in your publication 
reviewiug the various theories concerning the origin of the chalk flints. 
jNTo notice was then taken of a theory which, to my mind, explains the 
origin of flint better than any of those theories which have as yet been 
