No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 681 
logue of the external levator of the corresponding arch in 
Amia ; and muscle III the homologue of the corresponding 
dorsal interarcual ligament. It may here be stated that these 
latter ligaments in Scomber are still partly muscular. 
On the fourth arch, in Acanthias, there is no interarcuale I. 
As it is found, in Heptanchus, on that arch and the next fol- 
lowing ones, Vetter concludes that, in Acanthias, it has been 
absorbed, before the disappearance of its arch, by the ever 
advancing constrictor of the pharynx. It seems to me much 
more probable that it, either with or without other more pos- 
terior muscles of the series, has become the subspinalis of 
Acanthias and the retractor arcuum branchialium of Amia and 
teleosts. 
In selachians no ventral interarcual muscles are given by 
Vetter. He, however, describes them in Acipenser and in 
teleosts, and derives them, in both, from the interbranchial 
muscles of elasmobranchs, more particularly from the so-called 
interbranchiales of Chimaera. These latter muscles, although 
considered by Vetter the homologues of the muscles of the 
same name in selachians and teleosts, differ greatly from 
those muscles. They are found in the first three arches only 
of Chimaera, and arise, on each arch, from a posterior process 
of the pharyngobranchial of the arch, the surface of origin 
extending backward beyond the hind end of the process, and, 
on each of the first two arches, occupying part of the anterior 
edge of the epibranchial of the next posterior arch. The 
muscle runs the full length of its arch, lying on its outer sur- 
face, and is inserted on the outer surface of the hypobranchial 
of the arch. The muscle has, thus, almost exactly the origin of 
the interarcuales II and III of selachians, and the insertion 
of the obliqui ventrales of ganoids and teleosts. Furthermore, 
interarcuale II in Scymnus is said to extend outward and 
downward along the outer surface of its arch, to be inserted on 
the upper end of the ceratobranchial, thus closely resembling 
the interbranchiale of Chimaera. The muscle in Chimaera 
may accordingly be considered as representing a primitive con- 
dition in the differentiation of that deeper layer of the general 
constrictor to which Vetter assigns the interarcuales and adduc- 
