6 OLD RED SANDSTONE PISHES. 



The grounds upon which the division into Heterostraci and Osteostraci is based are 

 derived chiefly from the intimate structure of the cephalic shields, though there are also 

 other considerations in regard to the form of these shields which seem to warrant the 

 arrangement. It is remarkable that, while in nearly all the Osteostraci [Cephalaspis, 

 Auchenaspis, &c.) the orbits are large and placed near the centre of the shield, in the 

 Heterostraci ( Pferaspis Scaphasjm, &c.) there are either no orbits at all in the shield, 

 or they are minute and placed marginally. This distinction is not, howxver, persistent, 

 for in Eichwald's Thyestes {Cephalaspis verrucosus and Ceph. Schrenkii oi Pander) there 

 are, according to the figures given, no orbits excavated in the shield, although its structure 

 is similar to that of Cejjhalaspis. 



(a.) It will now be convenient to examine the question as to the structure of the shield 

 in detail, in order to exhibit fully the reasons wdiich have led to this division of the 

 CepltalaspklfB ; and, in so doing, I shall have to make most extensive use of Pro- 

 fessor Huxley's memoir on this subject. 



Professor Agassiz did not fail to perceive the differences in structure presented by his 

 species of Cephalaspis. He thus describes the constitution of the cephalic shield in C. 

 Lyellii : — " 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 disc 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," This view of the composition of the shield in C. Lyellii, I 

 need hardly say, has been shown to be erroneous. Professor Agassiz seems himself to have 

 changed his opinion ; for, in speaking of all four of his species together, he says — " 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 Pishes." — Money. Grcs Houye. He still seems to 

 have entertained the idea that the shield was composed of a number of minute plates, 

 though he abandons the notion of the existence of a subjacent osseous skull, and regards 

 the shield as a tegumentary ossification. 



The shield of Cephalaspis Lloydii Professor Agassiz described as consisting of an 

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

 Chondropterygious Pishes," and of an internal layer made up of superimposed lamellae. 



