678 ALLIS. [Vou. XII. 
external to the visceral arches. This seems confirmed by 
Kaestner’s description of the embryological origin of the skele- 
togenous elements of a segment, from the inner wall of the 
protovertebra of the segment (No. 62, Pl. XII), and by Kupf- 
fer’s positive statement (No. 71°, p. 110) that the cartilaginous 
arches in selachians arise from the mesoderm of the arch. 
That the muscles of the branchial segments lie primarily inter- 
nal to the branchial basket in Petromyzon (No. I11, p. 69, and 
No.) 713, Figs, ) has no bearing on the subject, that basket 
being of ectodermal origin and not homologous with the 
branchial arches of other fishes (No. 714, p. 118). 
The general constrictor of the visceral arches lying primarily 
external to those arches, the adductor muscle of each arch 
must aquire, secondarily, its position on the inner surface of 
that arch. This it does, in Amia, in two different ways, either 
by passing over the posterior edge of its arch, as on the fourth 
arch and probably the fifth also, or over its anterior edge, as 
on the mandibular arch. The innervation shows this conclu- 
sively, if it be not assumed that the nerve that innervates the 
muscle of one or the other of the arches has cut through the 
cartilage of its arch. 
Before further considering the adductores it will be better 
to seek the homologies of the interarcuales, those muscles 
being derived by Vetter, in selachians, from the same deep 
layer that gives origin to the adductores. 
In Acanthias there are, according to Vetter, three interar- 
cuales on each of the first three arches. They are called by 
him interarcuales I, II, and III. The first of these muscles, 
interarcuale I, arises on the upper, posterior process of the 
pharyngobranchial of its arch, and is inserted on the corre- 
sponding process of the pharyngobranchial of the next follow- 
ing arch. The second arises on the epibranchial of its arch, 
and is inserted at the base of the posterior process of the pharyn- 
gobranchial of its arch. The third arises with the second, on 
the epibranchial of its arch, and is inserted on the anterior 
process of the pharyngobranchial of the next following arch, 
crossing and touching the anterior end of the epibranchial of 
that arch. Between the second and third muscles, or, more prop- 
