80 



ZOOLOGY 



SECT 



which is the dentary (DNT). In Mammalia the dentary forms the 

 entire mandible, and articulates, not with the quadrate, but with 

 a large investing bone formed external to the latter, and known 

 as the squamosal (SQ). 



\ In the hyoid arch a replacing bone, the hyo-mindibular (HY. M), 

 appears in the cartilage of the same name, and ossifications are 

 also formed in the various segments of the hyoid cornua (EP. HY, 

 C. HY, H. HY, B. HY) and of the branchial parches (PH. BR, 

 EP. BR, C. BR, H. BR, B. BR). In the air-breathing forms 

 both hyoid and branchial arches undergo more^or less complete 

 atrophy, the whole gill-bearing apparatus becoming reduced mainly 

 to a small hyoid bone serving for the support of the tongue. 



SCP 



CL 



PU 



Flos. 776 and 777. Diagrams of the fore- and hind-limbs with the limb-girdles, actb. aceta- 

 bulum ; gl. gleiioid cavity ; p. cor. procoracoid ; I -V, digits. Replacing bones en. 1 , en. 2 , 

 centralia; COR. coracoid ; dst. 15 distalia ; FE. femur ; PI. fibula ; fi. fibulare ; HIT. 

 humerus ; It. ilium ; int. intermedium ; IS. ischium ; mtcp. 1 5 metacarpals ; mtts. 

 1 5, metatarsals ; ph. phalanges : PU. pubis ; RA- radim : ra. radiate ; SCP. scapula; 

 TI. tibia ; ti. tibiale ; UL. ulna; ul. ulnare. Investing b:>'i3 CL. clavicle. 



The skeleton of the msdian fins is formed of a single row of 

 cartilaginous rays or pterygiophores (Fig. 769, G and D.f.r.), lying in 

 the median plane, and more numerous than the vertebrae. They 

 may ossify, and may be supplemented by dermil fin-rays, of 

 varying composition, developed in the derm towards the free 

 margin of the fin. The latter are clearly exoskeletal structures. 



Both pectoral and pelvic fins are supported by pterygiophores 

 or radialia (Fig. 775, Rid.}, the basal or proximal ends of which are 

 articulated with stout cartilages, the basalia (Bus), often replaced 

 by bones, which serve to strengthen the fin at its point of union 

 with the trunk. 



