PECTORAL ARCH 



141 



Cm 



articulations being formed on the arch which fit into concave 

 facets on the fin. The point of attachment of the extremity may 

 be taken as separating the arch into an upper dorsal and a lower 

 ventral section. The former, which may exceptionally be con- 

 nected with the vertebral 

 column (viz., Raiidse), cor- 

 responds to the scapula, and 

 the latter to the coracoid plus 

 procoracoid of the higher 

 Vertebrata. 



In Teleosts and bony 

 Ganoids the secondary arch, 

 consisting of a row of bones, 

 forms the principal support 

 of the fin in the adult, the 

 main element being a large 

 clavicle. The arch becomes 

 secondarily connected with 

 the skull. (For further 

 details, cf. Fig. 100.) In 

 Dipnoans, clavicles and 

 supra-clavicles invest the 

 cartilaginous arch (Fig. 71). 



Co(Cl} 



JRa? 



Amphibians. In this 

 Class the pectoral arch shows 

 no direct connection with 

 that of Fishes, but is similar 

 in fundamental plan to that 

 of all the higher Vertebrates. 



It always consists on 

 either side of a cartilagin- 

 ous or bony dorsal plate 

 (scapula and suprascapula), 

 which curves round the side 



FIG. 100. LEFT PECTORAL ARCH AND Fix 

 OF THE TROUT. (From the outer side.) 



D 1 , IP, chain of secondary bones of the 

 pectoral arch (clavicle and supraclavicle), 

 which is connected with the skull by 

 means of the post-temporal (Cm) ; F, 

 S, bony fin-rays, shown cut away from 

 their attachments ; HS, bony ray on the 

 border of the fin which is connected 

 with the fourth basal element ; L, 

 foramen in scapula ; M 1 , metapterygium ; 

 Ra, Ra, the second and third, and 4, 

 the fourth basal element of the fin ; 

 Ra 1 , the second cartilaginous row of 

 radii ; S and Co(Cl), bony scapula and 

 coracoid, which have become developed 

 in the cartilage Kn. 



of the body and is con- 

 tinuous with two ventral 

 plates an anterior (procoracoid) and a posterior (coracoicC] (Figs. 

 101 and 102). The ventral part of the arch becomes connected with 

 the sternal apparatus. The humerus articulates with a concave 

 glenoid facet at the junction of the scapula and coracoid. The 

 two coracoid plates either overlap one another in the mid- 

 ventral line (Urodeles, Fig. 55, A, B and certain Anura e.g. 

 Hyla, Bombinator, Fig. 55, c), or else their free edges come into 

 apposition and unite (other Anura, e.g. Rana, Fig. 55, D). In 

 Anurans the procoracoid has a more transverse position than 

 in Urodeles, and comes into connection with the coracoid in the 

 mid-ventral line, thus giving rise to a fenestra between the two. 



