HYOMANDIBULA OF THE GNATHOSTOME FISHES 571 
quite unquestionably the same as in the specimen first described. 
In all three specimens the anterior portion of the muscle-sheet 
was wider and stronger than the posterior portions, and it was 
here, only, that the muscle fibers reached the middle line of the 
body. 
In the specimen first described a small and quite distinctly 
separate bundle of muscle fibers had their origins from the ventro- 
lateral edge of the hind end of the neurocranium, and, running 
directly posteriorly, were inserted on the anterior edge of the 
large muscle-sheet close to its mesial end. This little bundle 
of fibers was not found in the other two specimens, but in them 
certain of the anterior fibers of the large muscle-sheet were them- 
selves inserted on the ventro-lateral edge of the hind end of the 
neurocranium. 
This large muscle-sheet, considered as a whole, is thus at- 
tached, both anteriorly and posteriorly, to fixed, or relatively 
fixed (hyomandibula) points, and hence can not act, as a whole, 
either as a protractor or a retractor of the branchial arches. It 
can however act to draw the pharyngobranchials closer together 
and hence have either a protractor or retractor action on indi- 
vidual arches. It must also have a levator action on all the 
arches, for it is strongly attached to the neural axis. Thehyo- 
branchial portion of the sheet, the portion that lies between the 
hyal and first branchial arches, is certainly simply an anterior 
member of the Mm. interarcuales dorsales I of Vetter’s descrip- 
tions of other selachians, and it would seem as if the little anterior 
bundle of fibers found in one of the three specimens might be a 
remnant of a prehyal, or hyomandibular, portion of the sheet. 
The hyobranchial portion of the sheet would seem to be the homo- 
logue of the subspinalis muscle of Vetter’s and Marion’s (’05) 
descriptions of Acanthias, which has its origin on the hind end 
of the neurocranium, and its insertion on the dorso-mesial end of 
the first pharyngobranchial, while the small hyomandibular por- 
tion, found in one specimen, would seem to be the homologue of 
the subspinalis muscle of Vetter’s descriptions of Heptanchus, 
which has its origin on the ventral surface of the hind end of 
