6 The American Geologist. January, 1895 
radials, which fill the spaces thus left between the primaries. 
In some cases these interradials begin about the middle of 
the fin. These are not the rays of a second parallel series. 
They have no bases, but commence thin and narrow, gradually 
thickening and widening as they run out upon the front edge, 
where the}'^ attain their greatest breadth and entirely fill the 
gaps between the ends of the primary rays. Behind the tip 
of the fin these secondary rays taper to the edge, and between 
them occur tertiary rays, where the primaries and the sec- 
ondaries do not entirely fill the intervals. These are shown 
in the figure. 
The intercalation of these intermediate rays gives at first 
sight the impression of a bifurcation of the rays. But this is 
incorrect. There is no bifurcation. The primary rays con- 
tinue single to the extremities. The five foremost ones widen 
out to the margin, leaving no gaps. The others behind them 
remain of uniform width, or even become narrower outwardly, 
thus affording space for the secondary and tertiary rays above 
mentioned. These are the most conspicuous parts of the fins, 
and with the aid of the figure anyone will be enabled to rec- 
(^•gnize their general character. 
In regard to the basal portions, we can only regret that the 
crushed condition of the specimen precludes all certainty 
about its structure. Amid the confusion, one fragment is 
strongly suggestive of a primary, and another on the opposite 
side of a secondary, basal, from both of which the anterior 
fin-radials may be imagined to spring. But to enlarge on 
such possibilities would be little more than speculation and 
waste of space. It is safer to suspend our judgment until a 
specimen or a fragment shall come to light in which the struc- 
ture is well preserved, when the problem will be at once solved. 
Meanwhile it is safe to say that there is no proof of an 
archipterygial type in the form or structure of the fin. It 
more resembles an ichthyopterygium closely allied in form to 
that of the sharks of to-day and containing the same number 
of rays as do many or most of these. Important differences, 
however, are the absence of joints in the cartilages, their great 
length, and the smallness of the area of the fin that is occu- 
pied by the trichinosts. On all these points the specimens 
give positive evidence. 
