GEOLOGICAL SURVEY IN SILURIAN ROCKS OF SOUTH OF SCOTLAND. 843 



aculei is not a matter of age, for the smallest specimens of L. probletnaticus have slender 

 thorns, just as in adult examples. 



The condition of the caudal fin is here of prime interest, for on wetting the specimen 

 and examining it with a lens, the arrangement of its rays can distinctly be seen. And 

 so, if we are to judge from the analogy of other heterocercal fish-tails, the arrangement 

 here seen at once shows that the azygous row of scutes is on the ventral side of the 

 body. In other words, we see the body-prolongation distinctly passing into the longer 

 lobe of the fin, which is on the opposed side to the scutes, the rays of this lobe being 

 short, while those of the smaller lobe are long. 



Position and Locality. — Downtonian Beds at Slot Burn, Seggholm. 



Part II. — Results. 

 The Ccelolepid^e. 



The general form of the Ccelolepidae has now been ascertained. They are shark-like 

 fishes of comparatively small size, the largest example known being only fourteen or 

 fifteen inches in total length. The head, with the anterior part of the body, is depressed, 

 the pectoral fins are lappet-like, there is a strongly heterocercal caudal, but no other fins. 

 The dermal covering is seen in its most primitive form in Lanarkia, where it consists of 

 small hollow-pointed spines, open below, and without basal plate. It appears in a more 

 specialised form in Theloclus, where we have small shagreen-like scales constricted below 

 the crown, with a base more or less developed, in which there is usually an opening into 

 a central pulp cavity. Where their microscopic structure has been examined, these scales 

 are found to consist of simple dentine, with radiating tubules, with no Haversian canals ; 

 the crown is also covered with a layer of ganoine. No traces have been seen of jaws, 

 teeth, eyes, branchial openings, or internal skeleton. The last-mentioned part of the 

 organism must have been entirely cartilaginous. 



In the absence of hard circumorbital plates, of teeth, and of opercula, it is not sur- 

 prising that the position of the eyes, of the mouth, or of the branchial openings should 

 not be ascertainable, though in the Devonian Theloclus Pagei (xxxvi. p. 599) the position: 

 of the branchiae themselves seems to be indicated by certain transverse markings in the 

 broad and depressed anterior part of the fish. 



Of course the notion that the Selachian spines known as Onchus, or the teeth, which 

 have been named Monopleurodus and Anchistrodus, had anything to do with the 

 Coelolepidse, is now entirely disposed of. Nevertheless, looking at these fishes as a group 

 by themselves, and taking into special consideration the nature of their dermal covering, 

 we should have no hesitation in assigning to them a position among the Selachii, and, as 

 Selachians I classed them in my preliminary notice of the Lesmahagow fishes. 



In that notice I also applied to them the term " primitive." If that is so, then the 



VOL. XXXIX. PART III. (NO. 32). 6 



