G74 



Palaeontology, it is therefore evident, must day by day become 

 of greater importance. And the morphologist is coming to rely upon 

 fossil forms as the safest, if the most scanty and elusive testimony 

 as to the lines of derivation of structures and of groups. 



It becomes, accordingly, of very great interest in support of the 

 Thachee-Balfoue theory that the earliest known of typical sharks, 

 the group very generally regarded as not widely divergent from the 

 ancestral stem of the tishes, were unquestionably the possessors of a 

 fin fold type of paired fin. In Cladoselache, the shark of the Waverly 

 of Ohio (Lower Carbon, or by some. DevonianX whose characteristic 

 structures are now somewhat definitely known, there have been de- 

 scribed conditions in the paired fins which appear especially pertinent 

 to the archipterygial theory ; for not merely does this primaeval shark 

 present the most typical fin-fold characters in its paired fins, but it 

 invalidates the reference that has often been made to the archipterygial 

 conditions of Pleuracanths, sharks which were not merely later — 

 Permian, — but were in many ways highly specialized^). 



The object of the present paper is to direct especial notice to 

 the fact that a series of the fin fold type of paired fins actually existed 

 in the earliest forms of Elasmobranchs ; thus to furnish the long 

 needed historic evidence in behalf of the Thachee-Balfoue hypo- 

 thesis; to point, furthermore, out the range which the paired fins of the 

 Cladoselachids present — from the truly fin-fold type on the one 

 hand, to the spine-supported fin of the Acanthodian on the other; 

 and to suggest finally the stages in the evolution of these archaic fin 

 forms. It will not be attempted to trace further the relation of this 

 ptychopterygial form to the "monoserial archipterygium "', this having 

 been admirably outlined by Wiedeesheem *). 



The most fin- fold like of the known forms of paired fins appears 

 to the writer to be that of the ventral of Cladoselache fyleri, 

 Fig. 4. And this may be taken as a starting point for comparisons. 

 It certainly must be looked upon as representing a constricted remnant 

 of the continous lateral fold : it is nearly four times as long as wide, 

 supported from body wall to outer margin by transversely directed, 

 unjointed, metameraP) supports, ra dials, R., twenty-one in all. 



1) As specialized structures, judging from the studies of BEoyG>iAET, 

 DoLLO, Fbitsch and Jaeerl, we might include diphycercey, membrane 

 boneä of head roof, reduction in dermal defences, claapers. 



2) Gliedmaßen skelet, .Jena, 1892. 



3) Thev correapond in this region, approximately, with the number 

 of the myocommata, traces of which are shown in the fossils. 



