OF TRISTICHOPTERUS ALATUS. 391 
seen in Polypterus.* On the other hand, the pectoral limb in the Selachii, in 
the Sturionidz, Lepidosteide, and Amiadz, and in modern Teleostei, offers 
analogies with the heterocercal tail with shortened axis, the axis being abbre- 
viated, and the radials developed principally, or (in most cases) entirely, on one 
side of it. 
The ventral fins, whose rays are similar in structure to those of the pec- 
torals, are smaller than the latter, and are very slightly lobate at the base. 
They are better shown in one of the specimens from which Sir Puitip Ecerron’s 
figures were taken, than in any of the present suite. I have not observed the 
pelvic bones. 
The Internal Skeleton of the Trunk.—An osseous vertebral column is trace- 
able from the cranium to the extremity of the attenuated prolongation of the 
body through the upper part of the caudal fin. The bodies of the vertebree 
were certainly ossified, but whether they were completely so, or remained more 
or less in the condition of “ring-vertebre,” is not discoverable from any speci- 
men I have seen, as they are in every case so compressed that no view is 
afforded of their anterior or posterior surfaces. A series of distinct neural 
arches, passing upwards into spines, the latter flattened laterally in the fore 
‘part of the body, are seen appended to the dorsal aspect of the vertebral centra. 
Corresponding haemal arches and spines are seen in the caudal region, though 
anteriorly I have not been able to detect any trace of ribs, and the extreme 
caudal termination of the column seems to be formed by centra alone. 
The interspinous bones of the azygos fins have been well described by Sir 
Puitip Ecerton. Those of the first dorsal are small and obscure, but there are 
three very prominent ones supporting the second dorsal and anal fins respec- 
tively, and which are in turn supported above and below by a large flattened 
bony piece, considered by Sir Puitie EcEertTon to be probably a composite 
‘spinous apophysis, ‘‘ formed by the union of three or more spines.” Similar 
interspinous ossicles are seen supporting the lower lobe of the caudal, disappear- 
ing, however, before the extreme termination of the vertebral column is reached; 
their number is given by Sir Pure Ecerton as eight or ten. The ossicles sup- 
porting the anterior rays of the upper lobe of the caudal fin are certainly neu- 
rapophyseal in their nature. 
The Azygos Fins.—The first dorsal, placed opposite the ventrals, is small 
and narrow, being somewhat elliptic-lanceolate in shape, and frequently found 
more or less adpressed towards the back; the second is larger and of a more 
expanded triangular form, closely resembling the opposed anal in general pro- 
portions and form. The bases of all three are very short, and slightly lobate; 
their most anterior rays are short, but rapidly increase in length till the apex 
* The tail of Polypterus is not, however, absolutely diphycereal, though conforming more to that 
type than to any other. 
VOL, XXVII. PART III. 5K 
