FROM THE OLD RED SANDSTONE OF SHETLAND. 329 



portion of free surface, which latter is marked with ill-defined rugae, reminding us 

 somewhat of the ornament of a scale of Holoptychius. I have, however, no remem- 

 brance of having ever seen a Holoptychius scale with overlapped facets so sharply 

 marked off from the sculptured area, and ordinarily the overlapped portion of such a 

 scale forms an area of much larger proportional extent. 



Fig. 6 is a fragment concerning which still less can be said. It is an absolutely 

 irregular five-sided piece seen from the internal or concave aspect, but a certain amount 

 of a rugose ornament shows through from the outer surface. These rugse are arranged 

 in three groups or areoe, and are, within the limits of each area, parallel to each other 

 but at angles to those of adjacent arese, as is well seen in the figure. I cannot venture 

 any theory as to the identification of this fragmentary plate. 



The contour of the fragment shown in fig. 7 reminds us superficially of one of the 

 posterior ventrolaterals of Coccosteus, but there is no real evidence that this shape 

 represents that of the plate in its original entirety. At the upper left-hand corner, 

 however, where the surface is better preserved than over the rest of the specimen, 

 a few bright and smooth stellate tubercles are seen, which are not unlike those of a 

 Coccostean plate. 



EXPLANATION OF THE PLATES. 



[In these Plates all the figures are of the natural size.] 



Plate I. 



Fig. 1. Impression of outer surface of left posterior dorso -lateral plate of an undetermined species of 

 Asterolepis, the lower margin being deficient. For orientation see text-figure 1, p. 322. 



Fig. 2. Anterior part of a broken anterior dorso-lateral plate of Asterolepis from which most of the bony 

 matter has flaked off. For orientation see text-figure 2, p. 323. 



Fig. 3. Distal segment of pectoral limb of Asterolepis. 



Fig. 4. Articular element of proximal segment of pectoral limb of Asterolepis. 



Figs. 5, 6, 7. Undetermined fragments. 



Plate II. 



Fig. 1. Fragment of a large plate of Holonema omatum, Traq., showing the sculpture along the free 

 margin. 



Fig. 2. Counterpart of the same specimen. 



Fig. 3. Fragment of another large plate, with sculpture more of a reticulated pattern. 



TRANS. ROY SOC. EDIN., VOL. XLVI. PART II. (NO. 14). 50 



