GEOLOGICAL SURVEY IN THE UPPER SILURIAN ROCKS OF SCOTLAND. 887 



Fig. 4, Plate II. , shows one of the most complete specimens as regards the preserva- 

 tion of the external form. The rays of the caudal fin are also to some extent preserved, 

 giving ample corroboration to the demonstration of its heterocercal structure, previously 

 afforded by L. armatus. The same condition is still better shown in fig. 6, where the 

 division of this caudal fin goes into two unequal lobes, with fin -rays arranged after the 

 heterocercal manner, is perfectly obvious. 



Body muscles. — In fig. 5, a little way behind the post-cephalic rods, and decidedly 

 in the dorsal half of the body, a number of short, dark lines, passing obliquely upwards 

 and backwards, may be distinguished in the carbonaceous film which represents the 

 soft parts. The same phenomenon is also seen to a lesser extent in fig. 6 ; but in fig. 7 

 we have, in addition, a set of similar lines on the hsemal aspect, which pass obliquely 

 downwards and backwards. In this last specimen both sets are more complete, and 

 extend nearly the whole length of the fish. Of this phenomenon only two explanations 

 are possible. These parallel oblique lines represent either the neural and hsemal spines 

 of the vertebral axis, or the oblique septa of the myotomes of the body. The second 

 interpretation seems to me the most natural. 



Skin. — In addition to the median ventral row of aculeate scutes and the gridiron- 

 like arrangement of bony rods just behind the dorsal aspect of the head, there is some 

 evidence of the skin having been furnished with other hard parts, though not of such 

 a nature as to be ordinarily capable of preservation. This is seen in the specimen from 

 Birkenhead Burn, represented in fig. 8, which is a portion of the skin near the front of 

 the fish, in this case an unusually large example, — the position being indicated by the 

 post-cephalic rods, which stand out as prominent oblique black lines. On wetting the 

 specimen and examining it with a strong lens, the portion of skin in question is seen 

 to be obliquely crossed by dark, closely-set linear markings, parallel with each other, 

 and with the direction of the post-cephalic rods, below and behind which they are 

 situated. That these markings represent the remains of structures by which the skin 

 was strengthened cannot be doubted. 



'&■ 



Lasanius armatus, Traquair. 



This species was described by me in my former memoir from a single specimen, 

 from Seggholm, in the Collection of the Geological Survey of Scotland. Another 

 specimen, corroborating the first in all essential details, has been found since that time 

 in the same locality by Mr Adam Whyte, of Muirkirk. 



TRANS. ROY. SOC. EDIN., VOL. XL. PART IV. (NO. 33). 6 q 



