304 Elementary Biology. 



rod-like bones, the maxillae, which pass backwards along 

 either side. Both maxillae and premaxillae carry teeth. 

 The hoop is completed by a triradiate bone, the pterygoid, 

 which, with the assistance of a small bone, the quadrato- 

 jugal, unites the posterior end of the maxilla to the skull, the 

 whole being steadied by a hammer- shaped bone, the squa- 

 mosal, which passes from the point of junction of the 

 quadrato-jugal and pterygoid to the upper portion of the 

 ear- capsule. 



The mandible, or lower jaw, consists of two halves or 

 rami, each of which again is made up of four smaller bones. 

 The rami articulate with the quadrato-jugal on either side. 



There is also an arch (the hyoid), composed, partly of 

 cartilage, partly of bone, supporting the tongue and throat. 



The skull, and indeed the entire skeleton, of the embryo 

 is at first composed of cartilage only, part of which after- 

 wards becomes altered into bone, part remaining cartilaginous 

 during life. Not only in the amphibian skull, but also in 

 the skull of the higher animals, certain bones are developed 

 from the skin, membrane bones, either over the parts which 

 have remained cartilaginous or in addition to the cartilage 

 bones already formed. Many of the bones forming the 

 framework of the jaws are membrane bones. 



Turning now to the appendicular skeleton we note 

 first that it consists, us already stated, of two hoops or 

 girdles to which are attached the fore and hind limbs re- 

 spectively. These are known as the pectoral and pelvic 

 girdles ; both are composed essentially of six bones or their 

 cartilaginous representatives. The fore and hind limbs are 

 also built on the same type. 



Pectoral girdle and fore-limb. The pectoral girdle 

 consists of a median ventral bar of bone, the sternum, ter- 

 minated posteriorly by a plate of cartilage known as the 

 xiphi-sternum. Springing on either side from the median 

 line anteriorly to the sternum are a pair of strong bones, 

 the coracoids, and anterior to these another pair of more 



