PECTORAL ARCH 



107 



and outer circumference of the (primary) arch, convex articulations 

 being formed on the arch which fit into concave facets on the fin, 

 the point of attachment of 

 which may be taken as 

 separating the arch into an 

 upper dorsal and a lower 

 ventral section. The former, 

 which may exceptionally be 

 connected with the vertebral- 

 column (viz., Raiidas), cor- 

 responds to a scapula, and 

 the latter to a coracoid plus 

 procoracoid of the higher 

 Vertebrata. 1 



In Teleosts and Bony 

 Ganoids the bony (secondary) 

 arch forms the principal 

 support of the fin in the 

 adult, the main element 

 being a large clavicle. The Co(Cl) 



primitive relations are thus ^ 84 ._ LEFT PECTORAL AKCH 



much altered. The arch 

 becomes secondarily con- 

 nected with the skull. (For 

 further details, compare 

 Fig. 84.) 



Amphibia. In this 

 Class the pectoral arch 

 shows no direct connection 

 with that of Fishes, but is 

 similar in plan to that of 

 all the hio-her Vertebrates. 



FIN OF 

 THE TROUT. (From the outer side.) 



-D 1 , D 2 , chain of secondary bones of the 

 pectoral arch (clavicle and supra-clavicle), 

 which is connected with the skull by means 

 of the post-temporal (Cm) ; S and Co(Cl), 

 bony scapula and coracoid, which have be- 

 come developed in the cartilage (Kit) ; L 

 foramen in scapula ; J/ 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 ; US, 

 bony ray on the border of the fin which 

 is connected with the fourth basal element ; 

 F,S, bony fin -rays, shown cut away from 

 their attachments. 



It always consists on either side of a 

 cartilaginous or bony dorsal plate (scapula), 

 which curves round the side of the body 

 and becomes continuous ventrally with 

 two processes an anterior (procoracoid) 

 and a posterior (coracoid) (Figs. 85 A and B). 

 The ventral part of the arch becomes con- 

 nected with the sternal apparatus (com- 

 pare Fig. 43). The humerus articulates 

 with a concave glenoid facet at the junc- 

 tion of the scapula and coracoid. The 

 two coracoid plates either overlap one 

 another in the mid-ventral line (Uro- 



1 The pectoral arch of Dipnoans is intermediate in character between that of 

 Elasmobranchs and Ganoids. It shows so many special peculiarities as regards 

 form and position that it cannot be fully described here. 



FIG. 85A. DIAGRAM OF THE 

 GROUND TYPE or PEC- 

 TORAL ARCH MET WITH 

 IN ALL VERTEBRATA FROM 

 AMPHIBIA UP TO MAM- 

 MALIA. 



S, scapula ; Co, coracoid ; 

 Cl, procoracoid ; H, hu- 

 merus. 



