880 DR RAMSAY H. TRAQUAIR ON FOSSIL FISHES COLLECTED BY THE 



the Geological Survey. I have since that time had the pleasure of seeing the specimen, 

 and find it to be a very fair example of Thelodus Scoticus, in which the caudal fin is 

 exceedingly well shown. 



Order HETEROSTRACI, Lankester. 

 Family Coslolepid^e, Pander. 

 Thelodus Scoticus, Traquair. 



When I wrote my previous memoir, the general configuration of this genus and 

 species was known principally from specimens from the Downtonian beds of Seggholm, 

 Monks Burn, and Birkenhead Burn, those from the Ludlow rocks of Logan Water 

 being more useful for the study of the scales. Since that time, however, specimens have 

 turned up in the last-named horizon and locality which considerably add to our 

 knowledge of the species, and necessitate a correction in the definition of the genus. 



In Plate I. fig. 4 is represented a specimen of Th. Scoticus collected by Mr Tait in 

 the Logan Water beds, and showing essentially the same contour as those from the 

 Downtonian horizon, as may be seen by comparing figs. 3 and 4, Plate I. of my former 

 " Report." The fish is in front compressed vertically, and the outline of the lateral 

 flaps is well shown, while behind the tail is twisted so as to display a side view of the 

 caudal fin, of which the upper lobe is unfortunately wanting. The specimen looks 

 indeed short and stumpy compared with some others, but I need scarcely remind the 

 reader that in the absence of an exoskeleton of closely fitted plates, or of a well-ossified 

 endoskeleton, the contour of a fossil fish is extremely liable to variation. 



Dorsal Jin. — The absence of all fins except the caudal (unless the pectoral flaps are 

 to be looked upon as paired members) was in my former memoir and in my address to 

 the Zoological Section of the British Association at Bradford in 1900 given as a 

 character of the entire family of Ccelolepidae, but clear evidence is now forthcoming 

 that, in Thelodus at least, a small dorsal fin was present. It was Mr James Young, of 

 Lesmahagow, who first informed me that specimens of Thelodus Scoticus in his 

 possession had more fins than I had credited the genus with, and to his kindness I am 

 indebted for the privilege of figuring three specimens from Logan Water which show 

 the fin in question. 



These are represented in Plate I. figs. 1, 2, and 3, the dorsal fin being seen at d. 

 This fin is situated not far in front of the caudal, is small in size, rounded in contour, 

 and covered with minute shagreen scales of the same type as those of the rest of the 

 fish behind the head. I may here mention that, subsequent to my receiving these 

 specimens from Mr Young, Mr Tait showed me one of the same species from the 

 Downtonian of Seggholm in which the same fin was distinctly visible. 



Eyes. — In Plate I. figs. 5 and 6 are represented two heads of Thelodus Scoticus, 

 collected by Mr Tait from the Ludlow beds of Shanks Castle, Logan Water, in which, 



