VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 345 
there is no gill-opening, and no musculus constrictor super- 
ficialis is described in relation to this seventh arch. 
In the adult Chlamydoselachus I find conditions strictly 
similar to those in Heptanchus, excepting that in this fish 
there are but six branchial bars and six gill-openings, the 
sixth gill-opening lying anterior to the sixth branchial bar. 
The distal, ventro-lateral end of the so-called epipharyngo- 
branchial of the sixth arch lies close to the shoulder- girdle, 
and that bundle of the trapezius that has its insertion on that 
element of this arch extends nearly to its hind end. The 
insertion of the trapezius on the shoulder- girdle begins 
opposite the hind end of the sixth epipharyngobranchial, 
and from there extends upward along the anterior edge of the 
shoulder-girdle. The trapezius is overlapped externally by 
the dorsal ends of all of the constrictores superficiales, 
including the constrictor of the fifth branchial arch, and each 
of the five branchial constrictores is similarly overlapped by 
the next anterior constrictor, the muscles thus being, in this 
respect, serially homologous. 
In Acanthias and Scymnus the trapezius, as described by 
Abetter, resembles that in Heptanchus and Chlamydoselachus 
excepting in that its relations to the branchial bars are 
modified by there being but five branchial arches in these 
fishes, and in that the trapezius is here perforated by the 
slender tendons that are said to alone represent the dorsal 
portions of the four branchial constrictores superficiales. In 
the adult Mustelus the trapezius is said by Tiesing to closely 
resemble that in Acanthias, as described by Yetter, and 
Tiesing makes no mention of any fibres of the constrictores 
superficiales joining the trapezius, such as I find in my young- 
specimen of this fish. In Chimasra (Yetter, 1878), there are 
minor, and for my purpose, unimportant variations in the 
muscle. 
In Heptanchus and Chimsera, A r etter could not determine 
the "innervation of the trapezius, but Fiirbringer (1897) has 
since shown that in Heptanchus it is innervated by branches 
of the nervus vagus. In Acanthias and Scymnus the muscle 
