No. 3.] MUSCLES AND NERVES IN AMIA CALVA. hay, 
vated two muscles, a rectus superior and a rectus internus, and 
that the inferior branch of the nerve also innervated two 
muscles, a rectus inferior and an obliquus inferior. 
Turning now to the Cyclostomata we find that in Petromyzon 
the rectus internus arises near the front end of the orbit, and 
is innervated by branches of the oculomotorius which first pass 
through the obliquus inferior near the middle of its length. 
This manner of innervation renders it highly probable that the 
muscle could not have been derived from, or have given origin 
to, any of the recti muscles known in other fishes, for even if 
the muscle should acquire an origin with the other recti, the 
inferior branch of the oculomotorius would still lie wholly above 
it, a condition not known, so far as I can find, in any other 
animal. The position and innervation of the muscle both 
indicate its derivation, in Petromyzon, from the outer, under 
surface of a muscle or muscle mass giving origin to it and to 
the obliquus inferior. The embryonic condition of such a 
muscle mass is described by von Kupffer (No. 70) in Ammo- 
coetes. It is said by him to be the lateral and lower portion 
of the subcerebral part of the trabecular arch, that arch being 
the first, or most anterior, of the visceral arches. This same 
muscle, or muscle mass, must, in the prototype of the Gnatho- 
stomata, have given origin to the obliquus inferior and rectus 
inferior, the two muscles innervated by the inferior branch of 
the oculomotorius. The outer muscle of the two that so 
develop in the Gnathostomata must, however, have become 
the obliquus, instead of the inner one, as in Petromyzon ; and 
the nerve innervating the two muscles, in the Gnathostomata, 
must have run downward along the posterior edge of the mass, 
instead of through it, as in Petromyzon. The inner muscle of 
the two, the inferior rectus, as it separated from its companion 
and acquired an origin with the other recti or the hind end of 
the orbit, would then have pushed the inferior branch of the 
oculomotorius before it, keeping it always posterior to and 
below it, as it is always found. 
That the inferior rectus, in the Gnathostomata and higher 
vertebrates, arises as above supposed, is strongly indicated by 
the early development of the muscles of the eye in elasmo- 
