GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OE SCOTLAND. 837 



Position and Localities. — All the specimens figured are from the Downtonian Beds 

 of Seggholm, though there is also in the collection a fragment from the beds of similar 

 age at Birkenhead Burn. Mr Tait has also recently collected a few detached scales 

 from the Downtonian Beds' of the Pentland Hills. It is unfortunate that a fish of 

 such interest and importance should be so exceedingly rare, and the specimens as yet 

 obtained so comparatively fragmentary. 



Order ANASPIDA. 



The two remarkable genera to be now described are so unlike any other fishes 

 hitherto known that I feel under the necessity of erecting a new order for their 

 reception. And as one of these forms, Birkenia, presents some features reminding us 

 of Cephalaspis, we may as well, at least provisionally, place the order also in the 

 sub-class Ostracodermi. Nevertheless, the structure of the substance forming dermal 

 scales of Birkenia shows neither the bone-lacunse of the Osteostraci nor the dentine 

 tubules of the Heterostraci, but so far as I have been able to examine them micro- 

 scopically, nothing is seen but a homogeneous, or slightly fibrillated mass, though this 

 may possibly be the result of faulty preservation. 



Family Birkeniid^:. 



Small fishes, fusiform in shape, with deeply bifurcate heterocercal caudal fin, but 

 no paired limbs. Dermal hard parts in the form of scutes, which are in one form 

 nearly entirely absent. No cranial shield ; orbits, jaws, and internal skeleton unknown. 



Genus BIRKENIA, Traquair, 1898. 



Generic Characters. — Fusiform ; body covered with several longitudinal rows of 

 narrow scutes, arranged in lines running obliquely from above downwards and 

 forwards ; head bluntly rounded, also covered with narrow scutes ; an oblique row 

 of small round openings on the side just at the posterior boundary of the head ; 

 no orbit seen, and no evidence of cranial bones, jaws or shoulder-girdle ; no paired 

 fins ; caudal palaeoniscoid in shape, completely heterocercal, deeply bilobed and rayed ; 

 a small rounded dorsal situated far back, near the caudal. 



Birkenia elegans, Traquair. 



Plate V. figs. 1-4. 



1898. Birkenia elegans, Traquair, in Director-General's Summary of Progress for 1897, p. 73. 



Specific Characters. — Scutes finely tuberculated ; five rows on the sides of the 

 body, of which the upper two do not pass beyond the dorsal fin ; a row of six median 

 scutes between the anal region and the origin of the lower lobe of the caudal fin, each 



paper was presented to the Society, and also since the publication of my notes in the Geological Survey's Memoir on the 

 Silurian Boch of Scotland. 



