GEOLOGICAL SURVEY IN THE UPPER SILURIAN ROCKS OF SCOTLAND. 885 



are clear traces of the postorbital fossa (parietal organ of Rohon), as found in Cephalaspis, 

 but it is presently cut off behind by the edge of the stone, as seen in the figure. 

 Another specimen, not figured, shows, however, that the postorbital fossa was similar in 

 proportions and form to that of Cephalaspis, and that the same arrangement of radiating 

 vascular canals existed under the middle layer of the shield. 



Scales. — In figs. 1 and 2, Plate III., the scales are seen to be arranged on the flank 

 in nearly vertical rows, which, on approaching the middle line of the back, turn for- 

 wards at an obtuse angle, — whether they also change direction at the ventral margin is 

 not seen in any specimen. But their disposition on the sides and dorsally coincides 

 with the arrangement of the body-scales in Cephalaspis, save that in this case the long 

 narrow lateral and dorsal scales are represented by rows of separate rhombic scales. 



Posteriorly the scales become smaller, and on the caudal body-prolongation are 

 irregularly angular in shape. The dorsal and caudal fin-membranes are also covered by 

 minute scales, which, on the last-named fin at least, tend to a linear arrangement, as 

 already noticed in my previous " Report." 



To that " Report," p. 836, I may also refer for an account of the external sculpture 

 and internal microscopic structure of the scales ; here I take the opportunity of supple- 

 menting that description by a drawing, PI. II. fig. 10, which is so far diagrammatic 

 that it combines in one view the results obtained from more than one less comprehensive 

 vertical section. The three layers of the scale-substance are here very distinctly 

 shown, and special attention may be directed to the lacuna?, abundantly seen in the 

 sections of the ridges and tubercles which make up the outer layer. 



Observations. — The question first arises as to whether the specimens from which 

 the above description has been taken are strictly identifiable with those already adopted 

 as types of the genus and species. There can, I think, be no mistake with regard to 

 the originals of figs. 7, 8, 11, and 12 on Plate IV. of my previous memoir : the difficulty 

 is with regard to the original of fig. 6, which, as I have already stated {op. cit., p. 835), 

 shows no trace of the antorbital fossse, or of the postorbital valley, which are prominent 

 markings on the shield of Cephalaspis. As, however, there are clear indications of 

 orbits in that specimen, and as it conforms in other respects and in general appearance 

 to the new examples now figured, I am inclined to believe that the non-appearance of 

 these markings is due to the mode of preservation, namely, to extreme crushing, whereby 

 they have become indistinguishable, and that we are therefore justified in assuming 

 that one and the same animal is represented in all those examples. 



We have thus a creature whose general appearance reminds us, in the first place, of 

 the Coelolepidse by reason of its broad depressed head, which terminates on each side 

 behind in an obtusely round and apparently flexible "flap." But though there are no 

 cornua, the orbits and associated appearances are of the type characteristic of 

 Cephalaspis, and it is therefore clear that these flaps or lappet-like expansions in 

 question are the same with the structures in the last-named genus, originally considered 

 by Lankestee to be pectoral fins, and afterwards by Dr A. S. Woodward as being of 



