406 



THE GEOLOGIST. 



a gueSvS. We regard the whole as forming an abdominal plate, com- 

 posed of different 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 

 sucker might be attached could 



Kg- 3. 



not be torn from its place without the resistance of the other. 



COEEESPONDENCE. 



Origin of Flint Veijis in Cliallc. 



Dear Sir, — A short time since a paper appeared in your publicatiou 

 reviewing the various theories concerning the origin of the chalk flints. 

 No notice was then tnlxcn of a theory which, to my mind, explains the 

 origin of flint better than any of those theories hich have as 3^et been 



