392 DR TRAQUAIR ON THE STRUCTURE AND AFFINITIES 
of the fin is reached, from which they become gradually shorter, but increase in 
delicacy and in obliquity of direction—the most posterior ones being quite 
horizontal in position. The form of the caudal is indeed remarkable, presenting 
as Dr GUNTHER observes, “‘a most peculiar intermediate condition between the 
diphycercal tail of the Sirenide and the heterocercal of Dipterus.” It is large 
and fan-shaped, nearly truncated posteriorly, the hinder margin being only 
slightly excavated. A prolongation of the body axis, becoming very rapidly 
attenuated, and then tapering to a fine point, runs right through it, but as the 
lower margin of this prolongation slopes much more rapidly upwards from the 
commencement of the anterior rays of the lower part of the fin, its termination 
comes to the posterior margin of the latter much above the middle, so that, as 
in heterocercal forms, the larger part of the caudal fin is developed on the lower 
aspect of the vertebral column. The rays of the upper part of the caudal com- 
mence a little further back than those of the lower; in both cases they are 
short at first, but become rapidly longer till the upper and lower apices are 
reached, from which they become gradually shorter, finer, and more oblique in 
their origin from the body axis. ‘Those arising from the extreme point of this 
axis are very delicate, and project beyond the margin of the rest of the fin, so 
as to produce the appearance described by Sir Puitip Ecerton as “forming a 
kind of supplemental fin, projecting beyond the terminal margin of the true 
caudal fin.” It must, however, be observed that the rays in question, though 
projecting in that remarkable manner, form a perfectly continuous series with 
those of the rest of the caudal (see restored figure, Pl. XX XII. fig. 11). All 
these fins are composed of slender, closely-set rays, repeatedly dichotomising, and 
divided by frequent transverse articulations, the joints being, however, as in the 
case of the paired fins, always considerably longer than broad. Those of the 
anterior part of the lower lobe of the caudal are, as Sir Puitip EcErton has 
pointed out, stouter than the others; they are, in fact, the stoutest fin rays in 
the entire structure of the fish. There are no traces of fulcral scales on the 
anterior margins of the fins, and, in this respect, 7’ristichopterus differs from 
Gyroptychius and from the Saurodipterini, where such scales are present, 
though differing rather in form from the pointed imbricating fulcra of the 
Paleoniscide and Lepidosteide. The statement of Sir Partie EcEerron, borne F 
out by one of his figures, that the anterior rays of the upper lobe of the caudal 
are “short and /wcral, the anterior ones being short, and forming a marginal in 
Jringe along the upper edge of the fin,” I do not find corroborated by a very — 
beautifully preserved tail in the series of specimens from which the present f 
description is taken. The fin-rays overlap the extremities of the supporting — 
ossicles, and are more numerous than the latter elements—characters occurring q 
also in many other Crossopterygidz, and in the Palzeoniscide. 4 
Scales of the Body.—The scales are of moderate size, rounded, thin, and 

