14 VERTEBEATA phylum vm 



The gill apparatus (Fig. 2^5) of the Ganoidei and Teleostei does not differ 

 essentially from that of the Selachii, except that the gill rays supported by 

 the hyomandibular in the latter are replaced here by opercular plates, while 

 behind the hyoid there are never more than five (rarely four) bony arches, 

 and the branchiostegal rays of the hyoid arch become robust bony pieces or 

 plates. The hyoid is subdivided into an upper epihyal, a middle ceratohyal, 

 and a small hypohyal (basihyal) consisting of two pieces ; the median 

 connecting piece (copula) is extended more or less forwards into the tongue as 

 a glossohyal, and is sometimes covered with teeth, sometimes toothless ; a 

 second segment of bone belonging to the copula, directed backwards, is named 

 urohyal. The gill branches (branchialia) also consist each of three pieces (epi-, 

 cerato-, and hypo-branchiale), connected in the median line with copulae, and 

 beset throughout their entire length with delicate cartilaginous rays, some- 

 times also on their inner side with tooth-like prominences or spines. With 

 the epibranchials are also connected the short upper pharyngeal bones, which 

 often bear teeth. The last branchial arch is usually more or less modified, 

 and forms the lower pharyngeal bones. 



The limbs of fishes comprise not onl}?- the paired, but also the unpaired fins, 

 since both function as organs of locomotion. While, however, the latter are 

 exclusively dermal structures, the paired fins belong at least in part to the 

 internal skeleton, and correspond with the extremities of the higher Vertebrata, 

 the pectoral fins with the anterior, the pelvic or ventral fins with the posterior 

 limbs. 



Both the pectoral and pelvic fins are connected with originally cartilaginous 

 arches (pectoral and pelvic girdles), which may become ossified and covered 

 with bony plates in variable number. They are entirely absent only in 

 Amphioxus and the Cyclostomi. Except in the Selachii, the pectoral girdle is 

 usually fixed to the supraoccipital or squamosal bone. In the Selachii (Fig. 

 28, ^) it forms a simple arch of cartilage, ventrally closed, situated behind the 

 gills, its upper ends either terminating freely (sharks) or connected with the 

 vertebral column (rays) ; it is bent upwards at the insertion for the fins, and 

 pierced by nerve foramina. In many Ganoids (Chondrostei, Heterocerci, 

 Ci'ossopterj'-gii) the primary cartilaginous pectoral girdle, divided into two 

 pieces by a ventral suture, is covered on each side by three membrane bones, 

 of which the median and largest is termed clavicula, the lower one infra- 

 clavicula, the upper one supraclavicula. The latter is sometimes divided into 

 two pieces, of which the upper is named post-temporal. In the other Ganoids 

 and the Teleosteans the primary pectoral girdle is entirely replaced by ossifica- 

 tion. The girdle is here formed of two bones united by jagged suture on the 

 postero-internal face of the very large clavicle, the hinder element homologous 

 with the scapula, the antero-anterior element with the coracoid. In several 

 families there is also a third, slender, arched bar {Spangenstdck or precoi'acoid), 

 extending downwards and inwards to the lower end of the clavicle. 



The clavicle is always the principal bone of the pectoral girdle in the 

 Teleostei, and its size and form vary remarkably. A supraclavicle and a 

 post-temporal (P/) rest on its upper end ; while an ordinarily slender acces- 

 sory dermal bone, the post-clavicle, overlaps it behind. 



The pectoral fins themselves can only be homologised in part and with 

 difficulty with the anterior extremities of the higher Vertebrata. If we 

 accept, with Gegenbaur, the biserial fin of Ceratodus (Fig. 26) as the basal 



