ar 
THE MYOLOGY OF AMIURUS CATUS. 343 
limited, may explain the want of relation of the mvomere to it, but 
still one would expect to find the muscles in relation to parts situ- 
ated near it, 7.e., in front of the orbit. In the Selachians this is the 
ease ; the origin of the add. mand. is in these forms entirely in front 
of the eye, and its action is essentially that of a constrictor. It seems 
that there has been first of all a gradual passage backwards of the 
origin of the add. mand., (and also of the other trigeminal muscles), 
until in the Teleosts it has come to lie entirely behind the orbit, and 
that secondarily, there has been a downward growth of the muscle, 
so that the fibres have extended on to the hyomandibular, &c., the 
lowermost assuming a horizontal direction. The relations of the ori- 
gin of the add. mand. in the Cyprinoids, Perca and Esox, are in sup- 
port of this supposition. Vetter has pointed out that the add. mand. 
of the Cyprinoids is very much specialized, that of Perca slightly less 
so, and that of Hsox, to which Amiwrus is most comparable in this 
matter, more primitive than either ; and we find that in Zsom, the 
most primitive form, the muscle arises in part from the cranial bones, 
(viz., the pterotic and sphenotic), whereas in the others the origin has 
passed lower down. 
Why there should have been this passage backwards of the muscle 
to behind the orbit, it is rather difficult to say. Perhaps an explana- 
tion may be found in the fact that the muscle acts in the Teleosts 
more or less as a retractor of the mouth parts, justifying in this 
respect Owen’s designation of it as the retractor oris. If an upward 
movement of the mandible were all that was required, the arrange- 
ment which obtains in the Elasmobranchs would certainly be most 
effective, whereas, if retraction were also required, such a backward 
progression would be necessary. 
It may also be pointed out that since the muscle lies entirely 
behind the eyeball, the size of that structure will necessarily assist 
in determining the extent of the limitation of the origin to the 
hyoid arch. In Amiwrus where the eye is so very small, the origin 
persists much further forward than in any of the other forms 
examined, in all of which the eyeball is comparatively large. 
The adductor mandibule of the Teleosts has been derived from a 
constrictor muscle ; its relations to the hyoid arch have been produced 
by a necessity for its action as a,retractor oris; and the extent of its 
departure from its original position is partly determined by the size 
of the eyeball. 
