NECTURUS 85 



The lower jaw, or mandible, is a paired arch-shaped structure 

 composed on each side of three bones and a cartilage. The bones 

 are the following : the dentary, a large bone which forms the an- 

 terior and nearly all the outer surface of the mandible and bears 

 most of the teeth ; the splenial, a small plate of bone on the inner 

 surface of the mandible near its middle, which bears the hinder- 

 most teeth ; and the angular, which is on the inner surface of the 

 mandible, extending from its hinder end forward about two thirds 

 of its length. These bones are all membrane bones ; they overlay 

 the cartilage. This is Meckel's cartilage ; it forms the axis of the 

 mandible and is exposed to view on its inner side and at its hinder 

 end, where it articulates with the upper part of the skull. Meckel's 

 cartilage forms the mandible of the dogfish. 



The hyobranchial apparatus is an extensive cartilaginous struc- 

 ture which lies back of the mandible in the ventral body wall and 

 supports the tongue and the gills. The whole structure is com- 

 posed of four arches, — all cartilaginous, — the two foremost of 

 which are complete, the right and left halves meeting in the median 

 line and lying parallel with the arch-shaped mandible, whereas the 

 hindermost two are incomplete and lie parallel with the outer ends 

 of the foremost arches. A slender median cartilage joins the two 

 foremost arches, and a median rod of bone extends back from 

 the second one. 



The foremost of these arches is the hyoid arch; it supports 

 the tongue. It consists of a right and a left half, each of which 

 is made up of two bars of cartilage, — a short medial bar, the 

 hypohyal cartilage, and a long lateral one, the ceratohyal. 



The second arch is the first branchial, or gill arch ; each half is 

 composed also of two bars of cartilage,— a long medial bar, the 

 ceratobranchial cartilage, and the long lateral epibranchial. The 

 third arch is the second branchial arch. It is incomplete, the two 

 halves not meeting in the median line ; each half is made up of a 

 minute ceratobranchial and a long epibranchial, both of which lie 

 back of and parallel with the first epibranchial cartilage. The 

 fourth and last arch is the third branchial arch ; it also is incom- 

 plete, consisting, on each side, of a single epibranchial cartilage, 

 which lies just back of the second epibranchial. 



