570 EDWARD PHELPS ALLIS, JR. 
from this line of attachment across the median groove to the 
corresponding line on the opposite side of the head, thus enclos- 
ing and protecting the dorsal aorta. 
That portion of the muscle-sheet which lies anterior to the first 
pharyngobranchial, forms about one-half the length of the sheet, 
and, there being no pharyngohyal, the muscle fibers here all have 
their origins from a stout ligamentous band extending from the 
dorsal (proximal) end of the hyomandibula to the dorsal (proxi- 
mal) end of the first epibranchial, the band gradually diminish- 
ing in thickness and consistence toward its hind end. The pos- 
terior fibers of this part of the muscle-sheet have their insertions, 
as do the fibers of the interbranchial portions of the sheet, on 
the anterior edge of the next posterior pharyngobranchial, which 
is, in this case, the first pharyngobranchial. A small bundle of 
fibers immediately anterior to these posterior ones then passes 
mesial to the dorso-mesial end of the first pharyngobranchial, and 
its fibers, becoming tendinous, join and form part of the ligamen- 
tous sheet above described. The remaining fibers of this anterior 
portion of the muscle, which fibers form the larger part of it, are 
inserted in part directly on the ventro-lateral corner of the 
anterior portion of the vertebral column and in part on an 
anterior extension of the median subaortal membrane, the latter 
fibers reaching the median line of the head and there being in 
contact with their fellows of the opposite side. 
In the other two specimens that were examined the conditions 
differed from those above described only in that the muscle- 
sheet was narrower, and that the pharyngobranchials, excepting 
the first, extended entirely across it. Those bundles of fibers 
that, in the one specimen, passed mesial to the dorso-mesial ends 
of the pharyngobranchials were accordingly wanting in the two 
other specimens, excepting only the bundle that had its origin 
from the first pharyngobranchial. The related ligamentous sheet 
nevertheless existed in these two specimens as in the other one, 
and extended the full length of the branchial region, but it had 
unfortunately been cut and partly dissected away before atten- 
tion was called to it and its mesial and posterior attachments 
could not be determined. Those attachments were however 
