750 ALLIS. [VoL. XII. 
those structures in their arches. They seem, therefore, to be 
those elements of the hyoid arch, and the interhyal, or stylo- 
hyal, to be the epal element. 
The opercular bones and the branchiostegal rays, from 
12 mm. larvae upward, contain no trace of cartilage, and give 
no indication whatever of having been preformed in cartilage. 
They all lie external to the nerve and artery of the arch. They 
are, accordingly, probably entirely of dermal origin, and not 
homologous with the gill-rakers of selachians. 
31. In the mandibular arch the metapterygoid process of 
the metapterygoid and the anterior process of the same bone 
fulfil even better the same conditions. They seem, therefore, 
to be the supra- and infrapharyngeal elements, respectively, of 
the mandibular arch, and the quadrate to be the epal element. 
The palatine is then, probably, part of a premandibular arch, 
the coronoid process of Meckel’s cartilage being possibly an- 
other part of the same arch. 
32. The muscles of the visceral arches probably all lay 
primarily external to those arches, as a simple constrictor 
muscle. From that muscle the arcual and interbranchial 
muscles arose, by a separation of the simple constrictor into an 
inner and an outer layer. From the arcuals arose the adduc- 
tors and the interarcuals. The adductors, however, on the 
different arches, probably arose in different manners, and thus 
are not all strictly homodynamous structures. The same is 
true of the levators, which probably arose in part from the 
interarcual muscles, and in part from the interbranchials. 
33. The adductor mandibulae probably arose from a muscle 
resembling the so-called interbranchiale of Chimaera. That 
muscle in Chimaera is the simplest form of arcual muscle 
known, and, in giving origin to the adductor mandibulae, it had 
simply to slip over the anterior edge of the mandibular arch 
into its actual position on that arch. 
In Amia the adductor mandibulae is in large part a con- 
tinuous muscle extending from the upper ends of the hyoid 
and mandibular arches downward and forward, internal to the 
coronoid process of Meckel’s cartilage, into the hollow of the 
mandible. The muscle has become entirely tendinous in its 
