GEOLOGICAL SURVEY IN THE UPPER SILURIAN ROCKS OF SCOTLAND. 883 



What is of interest in this specimen is, in the first place, a dark line on each side, 

 which runs parallel with and internal to each of the right and left margins respectively, 

 at a distance of about •§- inch. In front these two lines are joined by a cross commissure 

 running behind the front of the head, which is oblique, owing to the deformation which 

 is frequent in these specimens ; this line is not distinct enough in the figures, being 

 only clearly seen when the stone is wet. At the sides we observe, between the margin 

 and the intramarginal line referred to, several transverse dark bars, which divide the 

 space concerned into a corresponding number of compartments, how many in all we 

 cannot say, as the fossil is cut off behind ; the counterpart, however, shows on one side 

 as many as six. 



On examination with strong lens the above-noted dark lines seem to be due to some 

 extent to a greater closeness of the minute dermal spinelets, which are black in colour, 

 with a lesser abundance of them on the paler interspaces. Be that as it may, 1 have 

 figured this specimen because of the manner in which these lateral markings recall to 

 our minds those on internal casts of the carapace in Cyathaspis integer (Kunth) and 

 Cy. Sturii, Alth., and which have been supposed to indicate the position of branchial 

 pouches. Geological Survey Collection ; collected by Mr Tait at Monks Water. 



Order OSTEOSTRACI, Lankester. 



Family Cephalaspim;, Traquair. 



Ateleaspis tessellata, Traquair. 



As indicated in my British Association " Address," and in the " Report of Progress " 

 for 1900, material has come to hand enabling us to form a much better conception of 

 the configuration and structure of Ateleaspis than was possible from the meagre 

 specimens available for my former memoir. It will therefore be necessary to furnish a 

 new description of the whole animal, from which it will appear that it is more closely 

 allied to Cephalaspis than I had previously imagined. 



General form. — In Plate III. fig. 1 a nearly entire specimen is represented, only the 

 tip of the tail being wanting. The head, here vertically compressed, while the tail is 

 twisted round so as to be seen in profile, is almost of an ovoid outline, rounded in front, 

 more gently so laterally, and assuming on each side behind a contour which, though 

 evenly rounded and with no tendency to angularity, recalls the " pectoral flap " of the 

 Coelolepidse. In fig. 2 the head is wanting, but the hinder extremity is better seen, 

 and shows that the heterocercal caudal fin was not, as I had taken for granted, bilobate, 

 as in Thelodus and Lanarkia, but unilobate, as in Cephalaspis. In both figures, but 

 likewise most distinctly in fig. 2, we see a small, dorsal fin of a rounded contour, 

 and occupying a position identical with that of the dorsal of Cephalaspis, namely, 

 opposite a space just in front of the caudal. 



Head. — As previously described, the head-shield of very numerous small polygonal 



