235 



The replacement or cartilage bones are the two occipitals 

 (around the foramen magnum and bearing the condyles), 

 the two pro-otics (ossified parts of the auditory capsule), 

 and the sphenethmoid. The others (except the quadrates) 

 are investment or membrane bones. The remainder of the 

 skull consists of persistent portions (cartilage) of the original 

 chondrocranium. 



Each half or ramus of the lower jaw (replacing Meckel's 

 cartilage) consists mainly of an articular angulo-splenial, 

 in front of which is an outer dentary, and a small mento- 

 meckelian at the chin ; the first two are membrane bones, 

 the third is a cartilage bone. The lower jaw articulates 

 with the upper parts of its own (mandibular) arch, namely, 

 the quadrate cartilages ; therefore the skull is autostylic. 



Give a Description of (1) the Vertebral Column, (2) the 

 Pectoral or Shoulder- Girdle and Fore- Limbs, and 

 (3) the Pelvic or Hip- Girdle and Hind- Limbs. 



(1) Consists of nine vertebrae, and a long unsegmented 

 portion, the urostyle. The first vertebra has two articular 

 facets for the condyles of the skull ; and it is without 

 transverse processes. The centra of vertebrae II. to VII. 

 are procoelous. The ilia of the pelvic girdle articulate 

 with the large transverse processes of the ninth vertebra, or 

 sacrum. 



(2) The shoulder-girdle has (on each side) the following 

 parts : a dorsal supra-scapula (partly cartilaginous) and 

 a scapula, which articulates with a ventral coracoid and a 

 smaller precoracoid (with associated clavicle). The last 

 two at their inner ends unite with the longitudinal carti- 

 laginous epicoracoids, which are fused together. In front 

 of the epicoracoids are the omosternum and cartilaginous 

 episternum, and behind are the sternum and cartilaginous 

 xiphisternum. At the junction of scapula and coracoid 

 is the glenoid cavity, for articulation of the arm (humerus). 



The fore-limb consists of humerus (upper arm), fused 

 inner radius and outer ulna (fore-arm), carpus (wrist) com- 

 posed of six carpals (two proximal, one central, three distal), 

 and hand with four complete digits and a rudimentary 

 preaxial digit (pollex or thumb). Each digit consists of a 

 metacarpal and (except thumb) two or three phalanges. 



(3) The hip-girdle consists of two long ilia, fused at their 

 posterior ends together with the small ischia and ventral 

 pubis ; the three form and surround the socket or acetabulum 

 (each side) for articulation of the leg (femur). 



