342 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
would persist to a greater or less extent. And so indeed they have 
done. In the Teleosts there are as representatives of the constrictors, 
the intermandibularis, the add. and lev. arc. pal., lev. and add. opere., 
the transversi dorsales and ventrales of the branchial arches, the 
interarcuales ventrales, etc. In these muscles the course of the fibres. 
is parallel to a plane at right angles to the axis of the body, and 
they act more or less as constrictors of the parts to which they are 
attached. The greater mass of the constrictors of the Selachians is 
in relation to the branchial cavity. Where the parts about the 
pharynx are comparatively elastic, constrictor muscles will be, of 
course, of great use in diminishing that cavity, and so forcing the 
water out through the gills; but when, on the other hand, the parts 
become less movable through ossification, other arrangements for the 
propulsion of the water appear. Membrane bones are developed to 
act as valves and protections to the gills, a portion of the constrictor 
musculature persisting, attached to them, and the lessening of the 
size of the pharyngeal cavity is produced by the elevation of certain 
parts in the floor of the mouth, and only slightly by the approxima- 
tion of the walls by constrictors. These latter, therefore, become 
limited to certain parts, instead of forming a more or less unbroken 
sheet over the branchial region. 
Bearing in mind the fact that in the head there were originally a 
number of myomeres, as represented by the head-cavities, which 
have been specialized into a number of distinct muscles ; and that 
to a very large extent the muscle fibres have lost their original 
direction, it is possible by means of the innervation to refer to their 
respective myomeres the various muscles. 
The Cranial Muscles.—Leaving out of consideration the muscles 
of the eyeball, which belong to a myomere or myomeres in front of 
the mouth, the first muscle segment to be considered will be that 
supplied by the fifth nerve. Belonging to this there is, in the first 
place, the add. mand., the fibres of which have, to a large extent, a 
longitudinal direction, and which extends between the mandibular 
and hyoid arches. Reasoning from analogy one would have ex- 
pected to find this muscle and those belonging to the same myomere 
extending between the first preeoral and the mandibular arches, but 
we find them in reality lying superficially to certain muscles sup- 
plied by the facial nerve. The development of the first preoral (or 
palatine) arch being in comparison with the succeeding ones so 
