No: 3:] J7ZOSCLES. AND NERVES [N AMIA CALVA. 529 
the superior oblique by the trochlearis, as it is in all fishes. 
This last muscle is developed from van Wijhe’s second myo- 
tome, and accordingly lies, primarily, posterior to all the other 
muscles. Its position, therefore, in Petromyzon, where it arises 
from the membranous hind wall of the orbit and is inserted on 
the under surface of the bulbus, probably represents a primitive 
condition. The position toward the front end of the orbit, 
found in all other fishes, must then have been acquired, as 
Firbringer suggests, by the gradual shifting of the origin of 
the muscle along the upper edge of the orbit. Nothing in the 
arrangement of the nerves of the orbit would have interfered 
with this change of position zf the r. ophthalmicus in Petromy- 
zon be the r. ophthalmicus profundus of other fishes, nor would 
the muscle, during the process, have been ever functionally at 
a disadvantage. What is apparently an intermediate stage 
in this shifting process is shown in Callorhynchus, where the 
muscle still arises from the upper edge of the orbit dorsal to 
the ophthalmicus superficialis, — the trochlearis, which inner- 
vates the muscle, lying, as it should, below that nerve. Other 
intermediate stages are probably presented by the Monotre- 
mata, for Goppert (No. 46) finds, in Echidna, fibres of the 
obliquus superior arising from the frontal near the Trochlea, 
that is, from about the place where the whole muscle arises in 
Callorhynchus ; and in both Ornithorhynchus and Echidna he 
finds the muscle arising less deep in the orbit than in other 
mammals. 
In Fig. 12, Pl. XXII, the arrangements of the muscles 
and nerves of the eyeball found in the several orders of the 
Ichthyopsida, and assumed to be found in the several proto- 
types of those orders, is represented diagrammatically. From 
the prototype of all vertebrates there are two lines of descent, 
the impulse leading to each being the splitting of the large 
inferior oblique. In one line, the outer of the two muscles 
thus formed becomes a rectus internus, and the earlier muscle 
of that name, innervated by the superior branch of the oculo- 
motorius, disappears, or fuses with the rectus superior, giving 
rise to the arrangement found in Petromyzon. In the other 
line, the inner of the two muscles formed becomes a rectus 
