530° ALLIS. [Vov. XII. 
inferior, and the earlier inferior rectus, innervated by the 
abducens, either fuses with the rectus externus, thus giving 
rise to a holocephaloid arrangement of muscles and nerves, 
or, if Huxley’s statement as to the innervation of that muscle 
be accepted, persists as the retractor bulbi, thus giving rise 
to the arrangement found in Urodela. Assuming this last 
arrangement to be the one first formed, there would arise what 
may be called a proto-urodelian type from which there are 
three lines of descent. One of these lines leads to the Holo- 
cephala and Plagiostomata, in both of which the retractor bulbi 
disappears, and in the latter of which the rectus internus 
acquires an origin with the other recti at the hind end of the 
orbit. A second line leads to the Ganoidei and Teleostei, 
where the retractor bulbi likewise disappears, probably by fusion 
with the rectus externus, as the frequently double muscle and 
double nerve in Amia indicate, and where a still further 
change is brought about by the formation of a new internus 
by the splitting of the rectus inferior. As this new muscle 
develops, the old internus disappears ; thus leaving but one 
muscle innervated by the superior branch of the oculomoto- 
rius. A third line leads to Amphibia and the higher verte- 
brates, the Dipnoi either arising from this line or from the line 
leading to elasmobranchs. The amphibian part of this third 
line separates into two parts, one leading directly to Urodela 
with but little change, and the other to Anura, where a new 
internus is formed, as in Ganoidei and Teleostei, the old inter- 
nus disappearing as in those fishes. In the higher vertebrates 
the arrangement is as in Anura, but there is often a levator 
palpebrae superioris, innervated, when found, by a terminal 
branch of the superior branch of the oculomotorius. This 
muscle thus seems to be developed from the disappearing 
rectus internus of the prototype, and the line leading to the 
higher vertebrates must accordingly lie between those leading 
to the two divisions of Amphibia, just as the line leading to 
Amphibia lies between those leading to the two great divisions 
of Pisces. 
In Ichthyophis a retractor tentaculi has been formed from 
one of the muscles of the eye (No. 108, p. 200), probably from 
