GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 829 



Genus THELODUS, Agassiz, 1839. 

 Syn. Coelolepis, Thelolepis, Pachylepis and Nostolepis, Pander. 



Generic Characters. — Form of the body as in the definition of the family. Scales 

 consisting of a base and of a crown separated by a constriction or neck. Crown of scale 

 round, oval, quadrangular, or sometimes acutely pointed behind, smooth or sculptured ; 

 base usually with an opening of greater or lesser size (sometimes absent) leading into 

 the central pulp cavity. 



Range in time. — Although Thelodus is a characteristically Silurian genus, its range 

 in time extends to the Upper Devonian, as Prof. Rohon has described and figured 

 scales of a species (Th. Tulensis) from strata of the latter age in Russia. I have also 

 shown that the " Cephalopterus " Pagei of Powrie from the Forfarshire Lower Old Red 

 belongs to the same genus (xxxvi.). 



Remarks. — Although the name Thelodus, given to the detached scales in the Ludlow 

 Bone Bed by Agassiz, implies that these little bodies are teeth, the laws of priority forbid 

 its alteration into Thelolepis, as proposed by Pander and adopted by Rohon. 



For a more detailed account of the history of the genus I must refer the reader to 

 my paper on Thelodus Pagei (xxxvi.). 



Thelodus Scoticus, Traquair. 



Plate I. figs. 1-10. 



1898. Thelodus Scoticus, Traquair, in Director-General's Summary of Progress for 1897, p. 72. 



Specific Characters. — Scales in front having the crown rounded, somewhat convex, 

 smooth, the edges distinctly crenulated, the crenulation passing down the neck as a sort 

 of fluting; base well developed with a conspicuous basal opening. Scales behind having 

 the crown acutely pointed posteriorly, and sculptured with several longitudinal ridges 

 and furrows. 



Note. — The anterior scales conform in general shape to the ordinary type of Thelodus 

 scales, but the crenulations are fewer than in any which I have seen figured. The 

 posterior scales resemble those of Thelodus (Coelolepis) Schmidti, Pander, in having 

 their crowns pointed behind and longitudinally sculptured, but judging from Pander's 

 and Rohon's figures, the sculpture in these scales appears to consist of incised lines 

 on a nearly flat surface, while in our present species the surface is convex and the ridges 

 are sharp and elevated. It is interesting to reflect that were Thelodus Scoticus only 

 known by detached scales, the two different forms of those bodies would certainly figure 

 as distinct species. 



Description. — Fig. 1 on Plate I. represents a specimen from the " Ceratiocaris 

 Band" of the Ludlow horizon in Logan Water, which, though it shows the upper lobe of 

 the caudal fin, is very deficient anteriorly ; this is the largest specimen known, as its 

 total length when entire would probably not be under 8^ inches. Fig. 2 is taken from a 



