XIII PHYLUM CHORDATA 515 



In tlie 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 deithrum, an 

 investing bone occurring in some Amphibia and Eeptilia (p. 302). 

 Adavicle 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-sca2mlar 

 segment. 



In the carpus there are four proximal bones — scaphoid, hcnar, 

 cuneiform, and pisiform. The scaphoid corresponds to the radiale 

 of the typical carpus (p. 83) ; 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 isnhia, 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 

 lone. In the shank the inner or tiiial element is always the 

 larger ; the fibula may be vestigial. A large sesamoid bone — 

 the patella — is almost universally formed in close relation to the 

 knee-joint. In the tarsus there are two proximal bones, the 

 astragalus and calcaneum, the latter undoubtedly corresponding to 

 the fibulare of the carpus of lower Vertebrates, and the proximal 

 part of the former to the intermedium and its distal portion to 

 the proximal of the two central elements present in the tarsus of 

 some Amphibia. The scaphoid or navicular represents the second 



