244 CLASSIFICATION OF VERTEBRATES. 



and quadrate ; or again, the pterygoid may be differentiated 

 into several elements (ento-, ecto-, meso-, and metapterygoid) ; 

 but in all cases the quadrate furnishes the support for the lower 

 jaw. The quadrate, in turn, is attached to the skull by a sus- 

 pensor apparatus formed of the hyomandibular alone ; or a 

 membrane bone, the sympletic, may intervene between quadrate 

 and hyomandibular. Thus this skull is hyostylic. The prim- 

 itive lower jaw (Meckel's cartilage) becomes incased in mem- 

 brane bones, of which a pair of dentaries in front are always 

 present, while an articulare and an angulare may be added on 

 either side. The roofing bones of the cranium may be a pair 

 each of parietals (small), frontals, and nasals. Beneath these is 

 a large fontanelle in the chondrocranium. In the chondrocra- 

 nium itself the following ossifications may occur : At the base 

 of the skull the four occipitals (basi-, ex-, and supraoccipital) ; 

 in the ear region five otic bones, the sphen-, pter-, and epiotic 

 above, the pro- and opisthotic below. Occasionally the opis- 

 thotic may be absent. Basi- and presphenoid never occur, their 

 place being supplied by the parasphenoid. AH- and orbito- 

 sphenoid are sometimes well developed, sometimes inconspicu- 

 ous or absent, while the ethmoid region bears three ossifications, 

 - a mesethmoid, and a pair of ectethmoids. 



The operculum is supported by the hyoid arch, the lower 

 portion of which, the hyoid proper, is connected with the hyo- 

 mandibular by a small interhyal bone. The operculum consists 

 of two portions, a dorsal, containing several flattened bones : 

 above, a preoperculum and an operculum proper ; below and 

 behind, a suboperculum and an interoperculum, all of which 

 may be regarded as modified branchiostegals of the hyoman- 

 dibular ; the ventral part of the operculum is supported by the 

 long and slender branchiostegals of the hyoid. The branchial 

 arches are united below by a copula composed of the fused basi- 

 branchials, which extends forward, connecting the series with 

 the hyoid. Frequently the fifth arch is modified into a tooth- 

 bearing accessory jaw, the so-called pharyngeal bones. 



The shoulder girdle is well developed. In the cartilaginous 

 arch two ossifications scapula and coracoid appear on either 

 side, and besides these a large membrane bone, the cleithrum 



