40 HUXLEY, ON THE SKELETON OF FISHES. 
corresponds in position with the interhemal cartilage, this 
cartilage is five or six times as large, and constitutes a broad 
vertical plate, longer than it is deep, and having its longest 
axis inclined downwards and backwards. Its superior and 
inferior margins are slightly excavated, the posterior is con- 
vex, the anterior deeply notched, so as to be divided into two 
processes, the anterior of which abuts against the inferior 
arch of the vertebra, while the posterior is applied against 
the posterior moiety of its under surface. On each side of 
the posterior convex edge of the cartilage (which they a little 
overlap), I found five slender osseous styles (4), the rudi- 
ments of the inferior caudal fin-rays. 
Immediately behind and above this anterior hypural apo- 
physis (as it may be termed) is another (’) very much smaller, 
vertical cartilaginous plate, which may be called the poste- 
rior hypural apophysis, having nearly the form of a right- 
angled triangle, and closely applied by its hypothenuse to the 
under surface of about the anterior two fifths of the free 
portion of the chorda. On each side of the posterior edge of 
this cartilage are three fin-rays (4), similar to those already 
described, so that in the caudal fin im this stage there are 
altogether eight double rays, and all these are imserted, not 
only below the notochord, but far in front of its termination. 
No neural arch is as yet developed from the terminal 
osseous ring. 
A great change had taken place in the tail of an embryo 
Gasterosteus, =;ths of an inch long (fig. 2). All the pre- 
ceding parts, however, were readily recognisable, notwith- 
standing their modifications. 
The penultimate centrum had become much longer in pro- 
proportion to its thickness, its superior and inferior arches 
were much more developed, and the latter sent down a spine 
independently of the interhzemal cartilage, around which a 
sheath of bone, which had coalesced above with the posterior 
part of the inferior arch, was now visible. The anterior 
hypural apophysis had become longer in proportion to its 
breadth, and was coated with a thin layer of bone. The pos- 
terior hypural apophysis had greatly enlarged both abso- 
lutely and in relation to the anterior, and traces of a bony 
deposit on its surface were discernible. The number of fin- 
rays had increased to fourteen ; of which two, very short, lay 
between the end of the interhemal cartilage of the penulti- 
mate vertebra and the lower angle of the anterior hypural 
apophysis ; six, gradually increasing in length, and becoming 
jomted superiorly, embraced the posterior edge of the in- 
ferior hypural apophysis; and six, of which the inferior were 
