GENERAL STRUCTURE. 



919 



(uh) is a vertically-compressed median bone, extending backwards 

 from the basihyals. Behind the hyoid arch occur the branchial 

 arches, the first of which consists of a median basibranchial, and 

 laterally from below upwards of a hypobranchial, ceratobranchial 

 (fig. 844, cb), epibranchial (eb), and pharyngobranchial. The latter 

 bones in the second and third arches are called superior pharyngeals, 

 and generally carry teeth. Finally, the gill-rakers (fig. 844, gr) are 

 spine-like bones attached to the inner margins of the branchial arches. 



In the mandible there is usually (fig. 843) 

 a dentary and articular piece ; but an angu- 

 lar, and more rarely a splenial or coronoid, 

 may also be present. 



In the appendicular skeleton, we find the 

 pectoral girdle of Ganoids and Teleosteans 

 consisting inferiorly of the primary cartila- 

 ginous elements corresponding to the scap- 

 ula and coracoid (fig. 844, sc, co), and 

 superiorly and laterally of a secondary chain 

 of bones developed from membrane, and 

 articulating superiorly with the pterotic 

 region of the skull. The bones of this 

 secondary chain are named from above 

 downwards posttemporal (fig. 844, pi), su- 

 praclavicular (scl), clavicular (cl), and a 

 postclavicular of two pieces (pel) ; while 

 there may be also an infraclavicular be- 

 low the clavicular. In Elasmobranchei 

 only the cartilaginous primitive girdle is 

 developed ; while in Dipnoi the girdle 

 is of very peculiar structure, and some- 

 what intermediate between that of Elas- 

 mobranchei and Teleostei. The pelvic 

 girdle is generally wanting ; but in the 

 Dipnoi there is a median cartilaginous 

 plate, with anterior and posterior paired 

 processes, of which the former are iliac, 

 and the latter give attachment to the hind 

 limbs. Elasmobranchs generally show a 

 degenerate pelvis of this type. 



The pectoral and pelvic limbs, or fins, are 

 so similar in structure that they may be considered together, 

 although the development of the latter is less specialised than 

 that of the former. No representatives of the arm and fore-arm 

 of the higher Vertebrates can be detected in Fishes, the basal and 

 radial bones or cartilages articulating directly with the pectoral 



vol. 11. c 



Fig. 845.— Skeleton of the left 

 pectoral limb of Ceratodus. Re- 

 duced. (After Giinther.) 



