5 14 ON CEPHALASl'IS AND PTERASPIS 



time the ridges of the cast would be very narrow, corresponding only 

 with the intervals between the summits of the ridges of the disk 

 sometimes 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 resem- 

 blances nor the differences in the structure of these two genera have 

 hitherto been rightly apprehended. 



The sole important differences consist, ist, in the absence of 

 osseous lacuna: in Pteraspis — their presence in Cephalaspis ; 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 

 examination 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 structure, 

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

 at their attached ends, and to send in longer or shorter reticulated 

 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 a^xoth of an inch, 

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

 necks is not more than uiu-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 



