94 DEAN. [Vot. IX. 
must be regarded as doubtful. They are, however, suggested 
as the writer has indicated in the diagram with dotted lines. 
Their fusion into a single plate would not prevent them sub- 
sequently from becoming conveniently separated or receiving 
distal increments where their axis comes to protrude from the 
body wall. » 
The basal joints of the foremost fin rays may be significant 
as the first appearance of jointed cartilage rays in a vertebrate 
extremity —their positions, flanked by an unshifting wedge 
of cartilage, might seem to suggest a reason for their flexibility 
and therefore for the origin of the joint. May not the condition 
here in its beginnings present the advantages of capacity for 
fusion and for flexibility, which will later be transmitted to the 
remainder of the fin, and give rise to the stage of its evolution 
figured by Wiedersheim? It would further appear that the 
inordinate forking, crowding and coalescence of rays in distal 
elements would naturally be most marked in the median and ~ 
hinder portion of the fin when the distal end of the axis 
(basalia) has come to be external. 
It is certain that in Cladoselache the fin rays (radialia) in their 
(unjointed) primitive character proceed directly from body wall 
to fin tip,—while in Xenacanthus and Pleuracanthus these 
have become jointed, often fused by lateral pressure, and 
reduced proximally to such a degree that more than half of the 
fin is dermal.t Derm rays have certainly, even in these early 
forms taken the duty of the marginal cartilage rays. It is 
significant that the cartilage rays in the posterior portion of the 
fin of Cladoselache become delicate and fork in their efforts to 
combine flexibility and lightness with strength. 
It is to be noted that this region when the fin margin becomes 
membraneous delicate striation may be seen,—these lines are 
parallel to the direction of the fin rays, pass between them, and 
represent perhaps, the beginnings of dermal rays which margin 
the fin in Pleuracanthus. 
The characters of the pectoral of Cladoselache, thus described, 
would accordingly represent a more primitive condition than 
1*Dermal’ is here used mainly for convenience, for in ontogeny these fin rays 
have been shown by Ryder to be mesodermic. 
