136 THE ANATOMY OF VERTEBRATED ANIMALS. 



the quadrate {Qu.) behind and below. Besides these there 

 may be three others: an external, ectopterygoid {Ecpt.), an 

 internal, entopterygoid {Ept.), and a metapterygoid (Mpt.). 

 The last envelops the upper and posterior portion of the 

 primitive quadrate cartilage ; and, fixing- itself against the 

 hyomandibular, contributes to the firmness of the union already 

 effected by the symplectic. 



Meckel's cartilage (MbJc.) persists throughout life, but the 

 ossification of its proximal end gives rise to an os articulare 

 in the lower jaw. To these an angular {An.) and a dentary 

 (-Z?.) membrane bone are commonlj' added (Fig. 47). 



The hyoidean arch is usually composed of two large oornua 

 — connected with the cartilaginous interval between the hyo- 

 mandibular and the symplectic by a stylohyal ossification, and 

 abutting, in the middle line below, upon one or more median 

 pieces, the anterior of which {entoglossal) supports the tongue, 

 while the posterior (urohyal) extends back to join the median 

 elements of the branchial apparatus. The cornua themselves 

 are usually ossified into four pieces : an upper (epihyaV) and a 

 lower {ceratohyal) large ossification, and two small ones (basi- 

 hyals) connected with the ventral ends of the lower large 

 ossification. 



There are usually five pair of branchial arches connected 

 by median ventral ossifications. The posterior pair are single 

 bones, which underlie the floor of the pharynx, bear no bran- 

 chial filaments, but commonly support teeth, and are called 

 hypopharyiigeal bones. In certain osseous fishes, thence 

 called Pharyyigognathi, they anchylose together into one bone. 

 The anterior four pair are composed of several joints, and the 

 ujapermost articulations of more or fewer of them usually 

 expand, bear teeth, and form the epipharyngeal bones. Sun- 

 dry important membrane bones are connected with the man- 

 dibular and hyoidean arches. 'Y\\q preoperculum, (P. Op.), oper- 

 culum (Op.), and branchiostegal rays, {Br.), already met with 

 among the Gfanoidei, are the most constant of these. Beneath 

 the operculum, lies a suboperculum {S. Op.), and below this 

 an interoperculum {I. Op.), which is connected by ligament 

 with the angular piece of the lower jaw, and is also united to 

 the outer face of the hyoidean arch. It may be altogether 

 tigamentous, as in the Siluroids. 



The branchiostegal rays are attached partly to the inner, 

 ind partly to the outer, surface of the hyoidean arch. They 

 support a membrane, the branchiostegal membrane, which 

 serves as a sort of inner gill-cover. 



