38 



OLD RED SANDSTONE FISHES. 



is u flattened oblong area, which is even a very httle concave, and may be named 

 the postorhital valley {p. o. v.). Quite anteriorly and near the margin or rim of the 

 shield two round masses are indicated by dotted lines {m. c). Like the double antorbital 

 hollow, they do not appear superficially, but are very constantly to be observed 

 in specimens, since the superficial [)ortion of the test is so rarely preserved. They 

 are really areas in which the concavity of the inferior aspect of the shield is not 

 maintained in its regularity, but increased between the two laminae. Li Euheraspis 

 (PI. XIII, fig. 12) they are very markedly developed all round the margin of the 

 shield, and are very conspicuous modifications of its structure. They may be termed 

 marginal cells. It may, perhaps, seem superfluous to designate each of these 

 parts by a distinct name, but it is of great importance that they should be clearly 

 recognised and distinguished, since they are present or are represented in all 

 genera of Osteoslraci, and were they better known in all tlie species would un- 

 doubtedly furnish valuable diagnostic characters by their modification and variation. 

 A glance through the plates will enable the reader to recognise some of them 



s Fig. 13. 



s superior lamina. i. inferior lamina. vi. margin. 



in nearly every specimen figured, whilst in most the crushing or fracture of the 

 specimen has obliterated some or other of them. The letters p. c. indicate the 

 posterior cornua, p. r. the posterior median ridge, p. s. the posterior spine, and p. a. the 

 posterior angles of the shield. It would be important to ascertain with what soft parts 

 these protuberances and cavities were connected, and what relation they may have to 

 similar structures in recent fish ; but into this inquiry, as well as that relating to the 

 ' central pit' in front of the disc of Cgathaspis and Pteraspis, I do not propose now to enter, 

 such an investigation being unnecessary in a descriptive monograph of fossil remains. 



Scales, Fins, and Form of the Body in Osteostraci. — The two specimens of 

 Ceplialaspis figured by Agassiz, in his 'Recherches sur les Poissons fossiles,' gave 

 some evidence as to the nature of the parts of the body in these fish, but our 

 knowledge on these matters has been very much extended by the discoveries of 

 my friend and coadjutor Mr. Powrie in Forfarshire, and by other 'finds' in the 

 same district. I do not at all suppose that the details ascertained with regard to the 

 genus Cephalaspis are necessarily true for all the allied forms ; indeed, the existence of 

 large dorsal plates in the subgenus Zenaspis clearly shows that they are not ; but as yet 

 the only Osteostraci (the only Cephalaspidce, indeed) of which we have remains of the 

 body (excepting a few scales of Fteraspis) are Cephalaspis Lyellii, C. Pagei, and C. 



