HYOMANDIBULA OF THE GNATHOSTOME FISHES 569 
course of the muscle fibers. Those fibers that have their ori- 
gins from the dorso-mesial (proximal) portion of a pharyngo- 
branchial, pass mesial to the dorso-mesial (proximal) end of the 
next posterior pharyngobranchial and are inserted on a series of 
long and slender tendons which run posteriorly, nearly parallel 
to the vertebral column. These tendons lie in and form part 
of a thin but strong sheet of ligamentous tissue which extends 
posteriorly considerably beyond the branchial chamber and is 
there attached to the tough fascia covering the ventral surface 
of the dorsal muscles of the trunk. The ligamentous sheet here 
lies between the trunk muscles and the constrictor of the oeso- 
phagus, but separated from the latter muscle by a median V- 
shaped muscle which has its origin from the ventral surface of 
the vertebral column and is inserted, on either side, on the 
closely adjoining dorso-mesial (proximal) ends of the fifth pharyn- 
gobranchial and the so-called sixth epibranchial (Garman, ’85), 
and in part also on the fourth pharyngobranchial. It is con- 
tinuous, along the greater part of its lateral edge, with the con- 
strictor of the oesophagus, and is evidently in process of differ- 
entiation from that muscle. I have not yet determined its in- 
nervation, but its position and its relation to the constrictor are 
such that it seems quite unquestionable that. it is innervated, - 
as that muscle is, by branches of the nervus vagus. This muscle, 
and not one or more of the intercuales dorsales muscles, is then 
quite certainly the homologue of the retractor arcuum, branchi- 
alium of Amia and teleosts, and Edgeworth’s (11) conclusion 
that this retractor muscle of Amia and teleosts is derived from 
trunk myotomes is probably in error. If this be so, one more 
of the instances frequently cited in support of the view that 
muscles are subject to radical changes in the manner of their 
innervation is apparently disposed of. 
The mesial edge of the ligamentous sheet above described is 
strongly attached, throughout its entire length, mainly to the 
ventro-lateral corner of the vertebral column, immediately lat- 
eral to a shallow median groove which lodges the lateral dorsal 
aorta, but in part, also, to the ventral surface of a tough mem- 
brane, continuous with the ligamentous sheet, which extends 
