8 



MAMMALIA 



CLASS V 



first all widely separated, arising from special centres of ossification, and 

 gradually expand until they meet, and are either connected by sutures or are 

 fused together. Thus the form and structure of the skull depend on the 

 manner of growth and union of the component bones ; and with reference to 

 the ontogenetic or phylogenetic succession, certain conditions are termed 

 primitive, and others more or less modified or specialised. Some of these 

 differences are indicated in the following table : — 



Primitive Characters. 



Bones united by sutures. 

 Brain cavity small and narrow. 

 Skull low, profile almost straight. 

 Facial portion of skall elongated. 



Nasal bones long, nostrils directed forward. 



Upper jaw low. 



Zygomatic arch complete. 



Orbit open behind, confluent with temporal 

 opening. 



Frontals and presphenoid dense or with small 

 air spaces. 



Bones on roof of skull smooth. 



Tympanic ring-like open below, free. 



Articular facet for lower jaw shallow, with a 

 postglenoid process behind. 



Halves of lower jaw united at the symphysis 

 by ligaments. 



Specialised Characters. 



Bones fused together. 



Brain cavity large, wide, expanded. 



Forehead region arched, or rising markedly. 



Facial portion of skull short or retreating, or 

 snout greatly prolonged. 



Nasal bones short or vestigial ; nostrils far 

 back or directed upward. 



Upper jaw high. 



Zygomatic arch interrupted or vestigial. 



Orbit closed behind. 



Frontals, and often adjacent bones, and the 

 base of the cranium, tilled with air spaces. 



Parietal, occipital or frontal bones having 

 prominent crests, protuberances, horns or antlers. 



Tympanic closed below, or with expanded 

 bulla ; with an auditory meatus ; and fused to 

 the periotic. 



Articular facet deep, and bounded by strong 

 ridges, postglenoid process wanting. 



Halves of lower jaw co-ossified at symphysis. 



The pectoral arch, in comparison with that of loAver vertebrates, has 

 sustained considerable reduction, consisting, as it 

 often does, of simply a scapula, and, in such forms 

 as use the fore limbs in climbing or for prehension, 

 of a clavicle. The coracoid and precoracoid are 

 present only in monotremes, being in all other 

 forms greatly reduced and fused with the scapula 

 as a process. 



The scapula (Fig. 7) is a rather large, rounded, 

 triangular bone, on the outer side of which is a 

 prominent crest {spina or crista scapulae, s) dividing 

 it into an anterior and posterior fossa. As a rule 

 the anterior end of the crest projects as an 

 elongated and bent acromion process {a). The lower 

 and narrower end is expanded to form a shallow 

 cup, the glenoid fossa {g), for the reception of the 

 head of the humerus. On the extreme end is a 

 short coracoid process (c), which ossifies from a 

 separate centre, and is united during adolescence 

 hy a suture only, finally fusing completely with 

 the scapula, 

 clavicle is a paired, slender, cylindrical, slightly curved bone, ex- 

 from the sternum to the acromion process of the scapula. It is 



Fio. 7. 



Right scapula of the dog. s, Scapular 

 crest ; a, acroniion ; c, coracoid pro- 

 cess ; cj, articular surface ; v, anterior 

 (coracoid) margin ; ft, posterior (glen- 

 oid) margin ; o, superior margin. 



The 

 tending 



