ON CEPHALASPIS AND PTERASPIS 503 



the one hand, and of C. Leivisii and Lloydii on the other, are thus 

 stated by Prof. Agassiz : — 



" In C. Lyellii the head is covered with a pavement of polygonal 

 plates, altogether similar to that which covers the head of Ostracion. 

 Each plate is convex in the centre, and is marked by radiating 

 grooves ending at the margin in denticulations, by which the scales 

 interlock. These scales appear to be osseous and to have their 

 external surface enamelled. At the circumference of the disk they 

 become confounded together, and the enamel presents wrinkles parallel 

 to the edge." Elsewhere these plates are said to be " true scales 

 juxtaposed." 



In the ' Recherches,' M. Agassiz describes " fibrous bones of the 

 head " under the " scales," and he particularly mentions and figures 

 the radiating direction of these " fibres ; " but in the ' Monograph of 

 the Old Red Sandstone Fishes ' I find the following general remarks 

 applied to the whole of the Cepkalaspidce : — 



" It would appear from the condition of the specimens preserved,, 

 that all the cranial bones were only protecting plates, which covered 

 a cartilaginous cranium similar to that of the Sturgeons ; at least 

 I have never been able to discover any cranial bones deprived of 

 that characteristic granulation, which indicates that the plates were 

 in direct relation with the integument. Therefore, I think there can 

 be no doubt that all these granular plates rested by their inner and 

 smooth surface on a cranial cartilage, such as is found in cartilaginous 

 fishes and in the embryos of osseous fishes." — Monog. Gres Rougi\ 



P- 3- 



Nevertheless, in speaking of the genus Cephalaspis, a few page.s 



further on. Prof Agassiz states that he has nothing to add to his 



previous account of the genus ; so that I am puzzled to know what 



view I ought to ascribe to him at present. We shall see by-and-by 



that the last quoted is the only one warranted by the facts of the 



case. 



The disk of Cephalaspis Lloydii is said to consist of an external 

 striated enamel, of a middle layer " composed of granules similar to 

 those of the bones of Chondropterygious fishes," and of an internal 

 layer made up of superimposed lamellje. Prof Agassiz considers that 

 this structure " singularly recalls that of the test of the Crustacea!' 



Notwithstanding these, partly real and partly imaginary, differences 

 between his different species of Cephalaspis, Prof Agassiz found in 

 Cephalaspis rostratiis (a species which I have had no opportunity of 

 observing) a form and structure of so transitional a character that he 

 included them all under the same genus. 



