NECTURUS MACULATUS. 419 
dermal bones connected with this arch are the vomer and the palato-pterygoid. These 
as well as the quadrate have been previously described. The ventral segment is 
represented by a very complete pair of Meckel’s cartilages, encased by three dermal bones, 
the dentale (dentary), spleniale (operculum), and angulare. The proximal, or posterior 
end of this cartilage forms a rounded articular surface which articulates with the mandib- 
ular process of the quadrate, and forms an element often ossified in amphibians, and then 
known as the articulare. 
2. The second visceral arch consists in the Selachians of a dorsal segment, the hyo- 
mandibular, and a ventral segment, the true hyoid. The latter piece is always present 
and well developed in Urodeles, and in Necturus consists of two well developed cartilages 
upon each side, but opinions differ widely concerning the fate of the hyomandibular. 
The views which connect it with the stapes of mammals and the columella of reptiles seem 
to have much support, but any connection between it and the Urodele operculum is evi- 
dently disproven by the origin of this latter piece from the side of the otic capsule. It 
thus seems safe to assert that the dorsal or hyomandibular segment of the second visceral 
arch has no skeletal representative in Necturus. 
3-5. These are the three gill arches which guard and regulate the two gill slits 
situated between them upon either side of the neck, support the three pairs of 
integumental branchiae, and furnish attachment for the muscles which regulate them. 
This association of external branchiae, belonging to the integumental system, with the 
three gill arches, which in fishes support the internal endodermic gills, has led in the past 
to a confused suggestion of homology between these genetically distinct structures. The 
gill arches are represented here by twelve cartilages, five on each side and two in the 
median line, and all of these pieces are joined with the four of the hyoid arch to form the 
hyobranchial apparatus. 
6. This is the rudiment mentioned above, and represented by an intermuscular 
septum. 
7. This arch is represented by a single pair of cartilages which guard the entrance 
to the trachea and extend along its sides; the cartilagines laterales, or laryngo-tracheal 
cartilages. 
The parts will now be taken up in detail. 
The mandible—The mandible consists of the four separate elements mentioned 
above :’ Meckel’s cartilage and the three dermal bones, dentale, spleniale, and angulare. 
The cartilage is well encased by the dermal pieces, but its surface is exposed along a part 
of the internal aspect, as well as over a large portion of the proximal, or articular end, 
where it forms the joint. In the larva the cartilages of the two sides meet at the mid- 
ventral line, and receive the protection of the dentalia upon their outer side alone, but as 
