828 Dll UAMSAY FI. TRAQUAIR ON FOSSIL FISHES COLLECTED BY THE 



as "Ludlow" and " Downtonian," the latter forming the uppermost part of the Silurian 

 system. 



The localities are — 



Ludlow — " Ceratiocaris" and " Pterygoids" bands in Logan Water. 

 Downtonian — Seggholm, Birkenhead Burn, Dippal Burn, Monk's Burn ; in the 

 Lesmahagow district. Lyneslie Burn in the Pentland Hills. 

 The fish-bands consist of hard, grey, flaggy shale, and in all the localities save Seggholm 

 the actual substance of the fossils seems preserved. At the last-mentioned locality, however, 

 the shale is somewhat decayed, and is in consequence soft, inclines to a brownish-yellow 

 coloration, and the scales and plates of the contained fishes are only seen in impression. 

 It will serve our purpose best to begin with the description of the fishes themselves 

 and of the facts concerning them, and thereafter to enter into the consideration of the bear- 

 ing of these new facts on certain previously obscure questions in palaeozoic ichthyology. 



Part I. — Descriptive. 



Order HETEROSTRACI, Lankester. 

 Family Ccelolepid^e, Pander. 



Head and anterior part of body flattened, trapezoidal, broader behind than in front ; 

 the sides forming posteriorly a right and left angular flap-like projection, the contour 

 of which, sharply marked off from the tail behind, is continuous or nearly so with the 

 lateral margin of the head. Tail narrow, provided with a deeply bifurcate and strongly 

 heterocercal caudal fin. The flap- or lappet-like projections probably represent the 

 pectoral members ; but there is no trace of ventrals, or of dorsal or anal fins. Dermal 

 covering, consisting either of minute shagreen-like scales having an internal pulp cavity, 

 and usually a hole in the base, or of minute pointed conical spines, without any basal 

 plate, hollow internally and widely open below. 



There is no trace of teeth or jaws or of any internal skeleton except in one instance, 

 the unique specimen of Thelodus Pagei of the Lower Old Red Sandstone of Forfarshire, 

 which shows certain markings, probably caused by a branchial apparatus (xxxvi. p. 599). 



This family was instituted by Pander (xxi. p. 64) for the genera Coelolepis, Pander ; 

 Thelodus, Agassiz ; Nostolepis, Pander ; and Pachylepis, Pander. Of these Nostolepis 

 and Pachylepis are synonyms of Thelodus (Rohon, xxviii. p. 31), while Coelolepis, in my 

 opinion, falls under the same category (xxxvi. p. 601). These genera were founded on 

 scattered scales, and nothing further has hitherto been known as to the creatures to 

 which these scales belonged. The Ccelolcpid scales have been considered to be " incertcr 

 sedis" (Smith Woodward, xxxviii. p. 157), or to have belonged to sharks (M'Coy, xvii. 

 p. 15; xviii. p. 576; Rohon, xxviii. p. 15 ; Rei3, xxvi. p. 211), or to Acanthodians 

 (Zittel, xliii. p. 30). Those who have referred the Ccelolepidas to sharks have also been 

 inclined to correlate with them the spines known as Onchus, which in England and 

 Russia often occur in the same beds with the scales in question. 



