GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 835 



of their tubercular ornament. Fig. 10 again represents a "squeeze" in modelling wax 

 taken from a similar piece of the counterpart, the tubercles appearing here in relief. 

 The tesserae become extremely small and at last indistinguishable on the parts corre- 

 sponding to the fin-flaps in the two preceding genera ; and it is also to be noted that 

 tubercles of a larger size are to be seen in impression along the lateral and anterior 

 margins of the head. 



About the middle of the head there are to be seen, on the counterpart, two small 

 crescentic markings, right and left, separated from each other by a space of one-third of 

 an inch, and situated half an inch back from the front, the convexity of each being out- 

 wards and the concavity inwards. These markings, indicated in the accompanying outline, 

 certainly do suggest the outer margins of a pair of orbits placed as in Cephalaspis, 

 and on no other supposition can I explain their presence, although there is no trace of 

 any inner margin to either, nor of any orbital space or opening, the impressions of the 



Fig. 2. — Outline sketch of the counterpart of the specimen of Ateleaspis tessellata, to show the 

 position of the crescentic marking alluded to in the text. 



tuberculated tessera? being continued all over the intervening surface. This may, how- 

 ever, be due to that vertical pressure which has reduced to absolute flatness a contour 

 which was no doubt originally more or less elevated or vaulted in the middle ; it must, 

 however, also be noted that there is no trace here of the ant-orbital fossae or of the post- 

 orbital valley which are prominent markings on the shield of Cephalaspis. 



On the tail the dermal covering assumes the form of rhombic scales arranged in 

 transverse rows, and the change from the polygonal tesserae to this condition is seen to 

 occur already in front of the posterior margins of the pectoral fin-flaps. In PL IV. fig 11 

 portions of three rows of these scales are represented, magnified three diameters, the 

 drawing being made from a " squeeze " in modelling wax taken from the counterpart. As 

 here exhibited, the sculpture of the surface consists of comparatively coarse, wavy, and 

 tortuous ridges, tubercles and furrows, which pass across the scale from before backwards. 



Fig. 7 represents a fragment apparently of the caudal part of a larger fish near its 



VOL. XXXIX. PART IIT. (NO. 32). 6 N 



