GEOLOGICAL SUKVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 851 



The greater part of the carapace, as seen from above, is composed of the oblong 

 median dorsal plate (D), which is slightly bilobate in front, while posteriorly it presents 

 a narrow deep notch, into which fits the dorsal spine (S). In front of the median plate 

 is the anteriorly -pointed rostral one (R) ; while fitting into the angle between these 

 two, on each side, is the triangular orbital plate (0), each of which shows a small opening, 

 just on the edge of the carapace, and supposed to be for the eye. Behind this is 

 the cornual plate (C), one on each side of the median dorsal, and forming a right and 

 left prominent postero-lateral angle. Each of these plates, also visible on the ventral 

 aspect of the fish, forms the sharp lateral edge of the carapace behind the orbital region, 

 and is perforated near its hinder angle by a pretty large oblique opening (B), usually 

 supposed to be branchial in its function. Lastly it is to be mentioned that on the 

 inner aspect of the carapace, just between the median dorsal and rostral plates is a 

 small but very distinct round median pit, which was probably supported by a minute 

 eighth or pineal plate. (See Lankester, xiv. p. 28 ; Alth, iii. p. 43.) 



Fm 7. 



Fig. 6. 



Fig. 6, Diagram of the dorsal surface of the carapace of Pteraspis rostrata, from specimens in the British Museum. 

 Fig. 7, Kestored outline of the same fish seen from the side, slightly altered from a figure by Mr A. Smith Woodward. 

 (D) Median dorsal plate ; (S) Spine ; (C) Cornual plate ; (0) Orbital plate ; (R) Rostrum ; (V) Ventral plate ; (B) Branchial 

 opening. The posterior caudal scales are here omitted. 



On the ventral surface (text-figure 7) is a large oblong median ventral plate (V), 

 once described by Lankester as a distinct genus (Scaphaspis). The mouth must 

 have been placed between the anterior margin of this plate and the posterior- ventral 

 aspect of the rostrum. 



It is impossible to compare this carapace with that of Drepanaspis (text-figure 5) 

 without being struck by the general resemblance in the arrangement of the parts 

 in both. In each there is on the dorsal surface a great oblong median plate notched 

 behind, although there is no dorsal spine in Drepanaspis. The cornual plates 

 of Pteraspis are represented by the postero -laterals of Drepanaspis, though in the 

 latter genus no branchial opening is observable, at least in the position in which it 



VOL. XXXIX. PART III. (NO. 32). 6 P 



