r 8 95 . THE PROBLEM OF THE PRIMAEVAL SHARKS. 41 



little cartilages — a kind of fin observed both in the living mud-fish, 

 Ceratodus, and in the Palaeozoic shark, Pleuv acanthus. The description 

 of the fin in question from the Lower Carboniferous of Ohio revealed 

 a still more primitive type ; that is, assuming (as most speculators do) 

 that fishes originally possessed on each side of the body a continuous 

 fold of skin, strengthened by parallel cartilaginous rods extending 

 outwards from the body-wall, this fold becoming sub-divided into the 

 pairs of pectoral and pelvic (ventral) fins as we commonly know them. 

 The researches of Dr. Bashford Dean (2, 3) now show quite clearly 

 what the paired fins of Cladoselache really are. They are mere 

 "balancers " with a more extended base-line than is customary. They 

 exhibit the series of parallel cartilaginous rods which once supported 

 the lateral fin-fold, practically unmodified in the pelvic fins, simply 



Fig. 2. 



-Cladoselache newbenyi ; lateral and inferior aspect, restored by Dr. B. Dean 

 Lower Carboniferous, Ohio. 



clustered and partly fused within the body-wall in the pectoral fins. 

 Dr. Dean, followed by Professor Cope (1), thinks that there is a 

 tendency in the pectoral fin for the hinder end of the row of basals in 

 the body-wall to rotate outwards — a process which would reduce the 

 point of attachment of the fin to what was originally its front angle. 

 The outwardly-turned row of basals would in this case correspond 

 with the median axis of the well-known paddle in Ceratodus and 

 Pleuracanthus ; and neither Dr. Dean nor Professor Cope sees any 

 difficulty in the development, quite secondarily, of a fringe of cartila- 

 ginous rays on the hinder border of this axis. As a result, these 

 American authors conclude that the modern tribasal or dibasal 

 shark's fin cannot have evolved from the paddle-like " archipterygium," 

 but that these two kinds of fin must have arisen independently from 

 the " ptychopterygium " (as Professor Cope aptly terms the arrange- 

 ment in Cladoselache). 



To recapitulate and discuss all the evidence for this theory would 

 lead here into too many technicalities, and we must be content to 



