LIMB-GIRDLES. 491 



intercentra) ; but all the processes become less and less conspicu- 

 ous towards the posterior extremity. 



The outer surface of the scapula is traversed by a ridge called 

 the spine which divides it into a prescapular and postscapular 

 portion. The spine usually projects at the lower end as the 

 acromion process, from which a backwardly directed process, the 

 metacromion, may arise. The anterior pair of extremities is 

 never absent. The clavicle is absent when the anterior limbs 

 serve only for the support of the anterior part of the body in 

 locomotion, or perform simple, pendulum-like movements, as 

 in swimming, walking, running, jumping, etc. (Cetacea, Ungu- 

 lata, Carnivora). Otherwise the acromion process of the scapula 

 is connected with the sternum by a more or less strong, rod- 

 shaped clavicle. The coracoid is almost always reduced to the 

 coracoid process of the scapula ; in the Monotremata only is it 

 a large bone which reaches the sternum/'' The posterior ex- 

 tremities are more firmly connected with the body than are the 

 anterior. In the Cetacea and Sirenia alone is the pelvic girdle 

 rudimentary, and is represented by two rib-like bones which are 

 quite loosely connected with the vertebral column. In all other 

 mammals the pelvic girdle is fused with the lateral parts of the 

 sacrum, and is closed ventrally by the symphysis of the pubis 

 and sometimes also of the. ischium (except in a few Insectivora). 

 The three bones are always united into one, the os innominatum, 

 which is pierced in the ischio-pubic region by a fontanelle called 

 the obturator foramen (corresponding to the coracoid fenestra 

 of the shoulder girdle of some lower types). The appendages 

 articulated to the pectoral and pelvic girdles are considerably 

 shortened in the swimming Mammalia, and either constitute, 

 as in the Cetacea, flat fins, the bones of which are immoveable 

 upon one another (in the Sirenia there is a joint at the elbow), 

 and in which there is a great number of phalanges, or, as 

 in the Pinnipedia, have the form of fin-like legs, which can 

 also be used in locomotion on land. In the Chiroptera (bats), 

 the anterior limbs present a large surface in consequence of an 

 expansion of the integument (patagium) uniting the limbs 

 with the sides of the body, and extended between the elongated 

 fingers. 



In the land Mammalia the extremities present considerable 

 variations both in their length and special structure. The 



