GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 831 



what higher level we have a second pair of ridges running parallel to the first, and also 

 meeting behind in a point, while, lastly, between them is a longitudinal median depres- 

 sion. The base is shallow, but I have not got a good view of its under surface. 



Observations. — The specimens from the Downtonian beds of Birkenhead Burn, 

 Seggholm, etc., though well preserved as to external form, do not show the configura- 

 tion of the scales in so distinct a manner as those from the Ludlow horizon in Logan 

 Water ; it is therefore not beyond the bounds of possibility that they may ultimately 

 turn out to belong to another though closely allied species. Meanwhile, to avoid 

 premature multiplication of names, I associate them with the Logan Water examples 

 which constitute the types of Scoticus. 



Position and Localities. — In the Pterygotus Band and in the Ceratiocaris Band in 

 Logan Water ; also in the Downtonian beds at Seggholm, Birkenhead Burn, and 

 Monk's Water. 



I may mention that I have lately, while examining some specimens of the well- 

 known " Bone Bed" from Ludlow, found one or two scales which resemble the posterior 

 ones of this species to a very close degree. 



Thelodus planus, Traquair. 

 Plate II. figs. 1-3. 



1S98. Tlielodus planus, Traq., in Director-General's Summary of Progress for 1897, p. 74. 



Specific Characters. — Upper surface of anterior scales round or somewhat oval, 

 slightly convex, smooth ; scales behind becoming narrow, elongated, and pointed pos- 

 teriorly, but without any strongly marked sculpture. 



Description. — Fig. 1, Plate II., represents the only specimen of this form which has 

 as yet been obtained. It measures seven and a half inches in length, but as the- 

 extremity of the tail is imperfect, it must originally have been at least two inches longer.. 

 The contour of the fish is pretty clear, although the edges are somewhat ragged ; the lower 

 lobe of the caudal fin is gone, while only the beginning of the upper one is preserved. 



The scales are very small, and are represented magnified eight diameters in figs. 2. 

 and 3 of the same plate. Unfortunately no view can be had of any of them, either from 

 below or from the side — all are either broken through or show the upper surface only. 

 Id those of the head (fig. 2) this upper surface is round or oval, smooth, ganoid, slightly 

 convex, and with an occasional tendency to crenulation round the edge, while in the 

 posterior scales (fig. 3) the exposed surface is more elongated, is pointed posteriorly, and 

 in some cases even a trace of a ridge on each side close to the margin may with some care 

 be made out. As shown in the figure, these scales seem to be somewhat irregular in size. 



This species is distinguished from Th. Scoticus by the smoothness of the posterior 

 scales, the species to which it is most allied being Th. glaber (Pander), from the Upper 

 Silurian of Oesel. 



Position and Locality. — From the Ceratiocaris Band in Logan Water. 



