378 OBSERVATIONS ON THE DEVELOPMENT OF 
that while the bony plates coalesce and form the inferior arches. 
of the caudal vertebra, the cartilaginous elements at their ex- 
tremities become the interhamal bones. The cartilage connected 
with the inferior arch of the penultimate centrum is therefore an 
“interhamal” cartilage. The anterior part of the inferior surface 
of the terminal ossification likewise has its osseous inferior arch (/) 
but the direction of this is nearly vertical, and though it is con- 
nected below with an element (4%) which 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 
convex, 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 (£), the rudiments of the inferior caudal 
fin-rays. 
Immediately behind and above this anterior hypural apophysis 
(as it may be termed) is another (2) very much smaller, vertical 
cartilaginous plate, which may be called the posterior 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. 
(k), similar to those already described, so that in the caudal fin 
in this stage there are altogether eight double rays, and all these 
are inserted, 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, zzths of an inch long (fig. 2). All the preceding parts, 
however, were readily recognisable, notwithstanding their modifications. 
The penultimate centrum had become much longer in proportion 
to its thickness, its superior and inferior arches were much more 
developed, and the latter sent down a spine independently of the 
interhemal 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 pro- 
portion to its breadth, and was coated with a thin layer of bone.. 
