324 PROCEEDINGS OF THE CANADIAN’ INSTITUTE. 
one will have a muscle passing from hyoid directly to the inferior 
pharyngeal, and, in addition, sending a slip from that bone to 
ceratobr. iv. ; and one can see in the slip of the obliquus ventralis 
of the fourth arch which passes forwards to the third, a homologue 
of the slip between ceratobrs. iii. & iv. in Amiwrus. The hyoid and 
branchial arches being the hemal arches of six of the vertebrae which 
enter into the skull, one may suppose that in ancestral forms there 
were sheets of muscle extending from one arch to the next, compar- 
able to the myomeres of the trunk ; or rather, since these arches are 
so early concerned in the function of respiration, it may be imagined 
that each head cavity developed into muscle above and below, but 
aborted in its median portion. We would then have on the under 
surface of the branchial arches a series of muscles passing from the 
hyoid to first branchial arch, from that to the second, and so on. 
Next, the inner fibres of these myomeres united to form a muscular 
belly extending from the hyoid directly to the fifth arch. The outer 
fibres did not take part in this modification, or at least only to a par- 
tial extent, certain of them becoming detached from their anterior 
attachment and united to the large belly, the posterior attachment 
persisting. The fibres passing to ceratobrs. i. and il. in Amiurus 
nigricans, are rudiments of these, and those to ceratobrs. iii. and iv. 
persisting examples. Those outer fibres which did not become modi- 
tied form the interarcual slips between ceratobrs. iii. and iv., and iv. 
and v. In other fishes the process of specialization has gone on still 
farther, certain slips becoming aborted and others losing their ori- 
ginal connections, so that the primary relations are lost. 
2. Muscuut InTERARCUALES OBLIQUI VENTRALES, (No. 38, Cuv.) 
On removing the preceding muscle, these \ Fig. t, Ob. V' and Ob. V’) 
are exposed. They are three in number in Amzwrus, and are small 
and triangular, extending from the hyobrs. i., il. and iii, to the 
ceratobrs. of the same arches. 
Action.—They draw the arches downwards towards the middle 
line and slightly forward. 
These may be considered as modified representatives of the inter- 
arcual slips between the third and fourth, and fourth and fifth arches. 
The original course of the muscular fibres of the myomeres is repre- 
sented by these latter, and since that of the fibres of the interarcua- 
les obliqui is almost transverse, they must have been transferred 
