EDWAKD PHELPS ALLIS. 
backward over the posterior edges of the dorsal and ventral, 
anteriorly-directed angles of the sigma ; and from the parts 
of the constrictor that cross or span these three angles are 
differentiated, respectively, the adductores arcuum branchi- 
al ium, the arcuales and interarcuales dorsales, and the 
coracobrancliiales of Dohrn's descriptions of embryos. These 
latter muscles are simply the proximal (anterior) portions of 
the ventral ends of the primitive constrictores of the branchial 
arches, they are of branchial origin, are innervated by 
branches of the nervus vagus of the arch to which they 
belong, and they coexist, in the adult, with the coraco- 
branchiales of Vetter's descriptions. The latter muscles are 
said, by both Dohrn and Edgeworth, to be derived from the 
ventral ends of the branchial myotonies, but their innerva- 
tion, in the adult, by spinal or spino-occipital nerves, their 
relations to the other hypobranchial muscles, and the marked 
want of accord in the descriptions of their development, all 
warrant the conclusion that they must be of spinal origin. 
The distal (posterior) portion of the constrictor muscle of 
each branchial arch of the Selachii, the so-called constrictor 
superficial! s, lay primarily not only on the anterior surface of 
the branchial rays of its arch, but also on that surface of the 
extrabranchials of its arch ; ^and, in the adults of recent 
fishes, its dorsal and ventral ends turn posteriorly, to a 
greater or less extent, across the dorsal and ventral edges, 
respectively, of the next posterior gill-pouch. When the 
constrictor contracted, the muscle was accordingly stretched 
across the extrabranchials of its arch, and certain of the 
muscle fibres, in certain fishes, were there cut in two by 
acquiring insertion on the extrabranchials. Other fibres 
simply became tendinous where they passed over the extra- 
branchials, and so there gave rise to more or less pronounced 
linear aponeuroses, or so-called septa. That part of each 
constrictor that lay between the dorsal and ventral extra- 
branchials of its arch thus became more or less cut out of 
the primarily continuous constrictor, and formed themuscuh s 
interbranchialis. This muscle is never found definitely 
