500 ZOOLOGY SECT. 



which enters into the glenoid facet ; but this very soon coalesces 

 with the scapula. The coracoid process, which is a separate ossifica- 

 tion in the young Mammal, and, though in most instances completely 

 fusing with the scapula and with the smaller coracoid element, is 

 sometimes recognisable as a distinct element up to a late period 

 (many Marsupials, Sloths), appears to correspond to the bone called 

 epicoracoid in the Prototheria (vide p. 505). In foetal Marsupials 

 the coracoid is represented by a well-developed cartilaginous element 

 which extends inwards and meets the rudiment of the sternum. 



In the scapula a spine is nearly always developed, and usually 

 ends in a freely-projecting acromion-process. It is developed, 

 unlike the main body of the scapula, without any antecedent 

 formation of cartilage, and is perhaps to be compared with the 

 cleithrum, an investing bone occurring in some Amphibia and 

 Reptilia (p. 293). A clavicle is well developed in many Mammals, 

 but is incomplete or absent in others ; its presence is characteristic 

 of Mammals in which the fore-limbs are capable of great freedom of 

 movement. In the embryo of the Theria there is, in the position 

 of the clavicular bar, a bar of cartilage, which coalesces with its 

 fellow in the middle line. The cartilaginous tract thus formed 

 segments into five portions a median, which coalesces with the 

 pre-sternum, two small inner lateral, which unite with the clavicles 

 or are converted into the sterno-clavicular ligaments, and two long 

 outer lateral, which give rise to the clavicles. The median and 

 inner lateral portions appear to correspond to the episternum of 

 Reptiles and Prototheria. An additional small cartilage may 

 represent the inner portion of the procoracoid of Amphibia. A 

 piece of cartilage at the outer end of the clavicle proper is sometimes 

 distinguishable the meso-scapular segment. 



In the carpus there are four proximal bones scaphoid, lunar, 

 cuneiform, and pisiform. The scaphoid corresponds to the radiale 

 of the typical carpus (p. 81) ; the lunar perhaps represents a 

 second centrale that occurs in some Amphibia ; the cuneiform is 

 probably the intermedium, and the pisiform the ulnare. 



The centrale is present sometimes as a distinct ossification ; the 

 five distal carpals are represented by the trapezium, trapezoid, 

 magnum, and unciform, the last being the equivalent of the fourth 

 and fifth distalia. There are never more than five digits, and in 

 many forms the number is greatly reduced ; only in certain Cetacea 

 does the number of phalanges in a digit ever exceed three. 



The three elements of the pelvic arch unite to form a single 

 bone, the innominate. The ilia unite by broad surfaces with 

 the sacrum ; the pubes, and sometimes the ischia, unite in a 

 symphysis. All three may take a share in the formation of the 

 acetabulum, but the pubis is usually shut out by a small cotyloid 

 bone. In the shank the inner or tibial element is always the larger ; 

 the fibula may be rudimentary. A large sesamoid bone the 



