390 DR TRAQUAIR ON THE STRUCTURE AND AFFINITIES 
to each side, and round the extremity, of a central scaled lobe, which is less 
than half the length of the fin, the latter beg thus subacutely lobate. In a 
specimen measuring 22 inches from the tip of the snout to the posterior margin — 
of the operculum, the entire pectoral fin measures 13 inch, and the lobe inch in 
length. The rays are slender and closely set, short at their commencement on 
each side of the lobe, but becoming very rapidly elongated; they commence to 
bifurcate very soon after their origin, and the process is frequently repeated, till 
at their terminations the longitudinal divisions are very delicate. They are also 
divided all along their course by transverse articulations; the joints being, how- 
ever, seldom less than four or five times as long as they are broad. The pectoral 
fin is not absolutely complete in any one example; a comparison, however, of its 
appearance in the various specimens of the suite shows that it would have pretty 
much the form shown in the restored figure given in Pl. XXXII. fig. 11. The 
lobe is covered externally with scales similar to those to be presently described 
as covering the body, though smaller, and which generally completely obscure 
the supporting skeleton within. Nevertheless, in the specimen represented in 
fig. 9, some insight is derived into the nature of this internal skeleton, though, 
unfortunately, not to so full an extent as might be desired. There is first, at 
the lower and posterior part of the clavicle (cl), some obscure bony matter (a), 
which may possibly represent the remains of ossification in the scapulo-coracoid 
cartilage. This is followed by a central basal stem for the fin, consisting of at 
least two flattened oblong pieces (4 1, 6 2) articulated end to end. On one side 
(from the possible twisting round of the otherwise displaced fin, it is hard to — 
say if it really be the medial or lateral) are two distinct radials (r 1, 72) set at 
acute angles to the axial stem, and of which the second arises at the place of 
junction of the first with the second axial segment. Attached to the extremity 
of the second axial segment are two additional pieces, one of which (4 3) seems 
to represent a third division of the axis, the other (7 3), slightly diverging, may 
be considered as a third radial. There is no clear evidence of radials on the 
other side of the axis, but as fin rays arealso here present it is hard to suppose — 
that they were destitute of similar supporting elements, and it seems meanwhile ~ 
probable that defective preservation is the cause of their not being distinctly 
recognisable. Were radial elements present on both sides of the axis, the — 
skeleton of the pectoral of Tristichopterus would present an abbreviated form _ 
of that of the Ceratodus limb, in which we have an elongated segmented central 
axis (Archypterygium of GEGENBAUR) set with segmented radials on both sides. — 
Dr GtnrueR has suggested an analogy between the doubly fringed acutely — 
lobate pectoral of Ceratodus, and the diphycercal tail with elongated axis in 
that genus, and many fossil ones; the pectoral of Tristichopterus, and probably — 
of all other Crossopterygide with subacute and obtusely lobate structure, — 
would in like manner correspond with the diphycercal tail with shortened axis — 






