1858.] HUXLEY CEPHALASPIS AND PTERASPIS. 277 



tvith the intervals between the summits of the ridges of the disk, some- 

 times broad, and corresponding with the intervals between their bases. 



Comparison of Pteraspis and Cephalaspis. 



If the exposition which has just been given of the structure of 

 Cephalaspis and Pteraspis be correct, it follows that neither the 

 resemblances nor the differences in the structure of these two genera 

 have hitherto been rightly apprehended. 



The sole important differences consist, 1st, in the absence of 

 osseous lacunae in Pteraspis — their presence in Cephalaspis j 2nd, in 

 the different general character and arrangement of the vascular 

 sinuses ; 3rd, in the different mode of arrangement of the external 

 layer. These differences appear to me to be in themselves fully suffi- 

 cient to warrant a generic distinction, but not more ; for they are 

 not greater than may be found among closely allied genera. 



It will be observed that the account of the structure of Pteraspis 

 given by M. Kner coincides, so far as it goes, with mine ; and the ex- 

 amination of one of his Pteraspides (of which Sir Philip Egerton, 

 with his usual liberality, has permitted me to have a section made), 

 though not so satisfactory as I could have wished, still leads me to 

 entertain no doubt that his fossils are really Pteraspides, and closely 

 allied to Pteraspis Lloydii. 



In this specimen, however, the histological characters which have 

 been described are almost all undistinguishable. All that remains 

 of the Pteraspis is a yellowish substance, without any definite struc- 

 ture, which appears in the section to form loops broader at their free 

 than at their attached ends, and to send in longer or shorter reticu- 

 lated processes of a similar character into the interior of the matrix. 

 The interspaces of the loops are filled up with crystalline masses of 

 carbonate of lime (?) . 



The length of the loop-like processes is about ^jo"th of an inch, and 

 the breadth of their wide end about the same ; the width of their 

 necks is not more than -g^th, or thereabouts. 



Now these are, as nearly as may be, the average dimensions of the 

 sections of the ridges of Pteraspis. 



No one can, I think, hesitate in placing Pteraspis among Fishes. 

 So far from its structure having " no parallel among Fishes," it has 

 absolutely no parallel in any other division of the animal kingdom. 

 I have never seen any Molluscan or Crustacean structure with which 

 it could be for a moment confounded. Its relations with Cephalas- 

 pis, on the contrary, are very close. In each the shield is excessively 

 thin, and composed of three or four layers: — 1st, an "internal," 

 composed of lamellae parallel with the surface, and traversed more or 

 less obliquely by vascular canals ; 2nd, next to this is a "middle layer," 

 containing the network of wide canals or areolae ; 3rd, the " reti- 

 cular layer," described in Cephalaspis as part of No. 2, from which 

 it is not distinctly marked in that genus ; 4th, the " external layer," 

 consisting of a cosmine-like substance raised into ridges or tubercles. 



The " bony granules," or "prisms," supposed to be characteristic 



