A TELEOSTEAN FISH. 259 
mentioned has similarly a thick, well-ossified, posterior edge and 
proximal portion, whilst the rest 1s very thin and membranous. 
The dorsal and ventral hypurals, which give attachment to the 
majority of lepidotrichia, are now widely divergent, thin, and 
membranous. These two, together with the hypural immediately 
in front, are ankylosed to the last centrum. Of the two epiurals 
whose development I have traced above, the posterior one is well 
ossified and thick, while the anterior one has a thick, well-ossified 
posterior edge and a thin, membranous, expanded anterior edge. 
The uptur ned tip of the notochord is quite free and naked, 
reaching about halfway up to the anterior margin of the dorsal 
hypural (d.). The upturned nerve-cord also runs up to about 
this level, beyond which J have failed to trace it in the dissection 
T have made. The cartilaginous elements to which the fin-rays 
ave attachel have grown a good deal by this time, but have not 
become ossified. 
SUMMARY. 
1. The development of the vertebral column begins at the 
caudal end, the hypaxial elements being the on to appear. 
This eoincides with a down-bending of the notochord (compare 
with this the analogous condition of the vertebral column in the 
Ichthyosauria). Epaxial elements do not appear until this con- 
dition has given way to the straight condition again. Arches 
appear as paired cartilages at the sides of the caudal arter y and 
vein and of the nerve- weed They are separated from the 
notochord by connective tissue. 
2. The notochord is of a relatively enormous size, and persists 
with only slight constriction throughout life. The centra consist 
of thin papery lamelle of membrane-bone. Ossifieation is 
generally weak. 
3. The neural and hemal arches of the penultimate centrum 
are double, owing to the splitting of single rudiments. Similar 
phenomena are to be observed in other fishes. ‘They may be 
produced by different causes :— 
(1) Splitting of an originally single rudiment. 
(2) Crowding of two arches on to one centrum. 
(3) Fusion of the first epiural apophysis with the last neural 
arch. 
(4) Exceptional equal development of both arch and inter- 
calary. 
(5) Secondary diplospondyly, ¢. e. fusion of two centra. 
4. Large cartilages are present above and below the last two 
centra in the adult, which support a dorsal and a ventral series of 
procurrent fin-rays. Their great size is probably connected with 
the weakness of general ossification. 
5. The hypural bones of the adult are formed by a fusion of 
hemalarchesand radials. This compound nature of the hypurals 
Proc. Zoou. Soc.—1914, No. XVIII. 18 
