296 



VERTEBRATE SKELETON 



and the olecranon is large, the distal end of the ulna being the smaller. In 

 recent species there is a scapho-lunatum with which the centrale unites in the 

 adult. Rudiments of the first digit are always present, (smallest in Hycena) 

 even when the first metacarpal is rudimentary, the digit itself is reduced in 

 Felidae and Canidse. The femur has no third trochanter and the crural bones 

 are separate. The articular surface of the talus is deeply excavate. Dogs 

 (except some domesticated races) and cats have a vestigial hallux; the dachs- 

 hund has a double great toe. 



The Pinnipedia have short stylopodial and zeugopodial bones and no entepi- 

 condylar foramen, and the femur lacks a third trochanter. The carpus (fig. 319) 

 and tarsus are much like those of Fissipeds. The hind feet are rotated backwards 

 so that they are nearly parallel with the body axis, the outer 

 digits of the feet being longer than the others. 



Cetacea have practically lost the hind limbs; the anterior 

 are modified into swimming flippers, short in Mystacocetes, 

 long in Denticetes, especially long in Megaptera. With this 

 there is a great reduction in the length of the separate bones. 

 The entepicondylar foramen is lost, radius and ulna are 

 parallel and similar, sometimes partly united. The carpus 

 in pentadactyle Denticetes is much like that of normal mam- 

 mals and may have two centralia. Some have five separate 

 carpalia, but usually there is fusion and loss, so that dolphins 

 with four digits have three carpalia (some say these are 2,3,4, 

 others that 3 is lost, i persisting). There are four or five 

 digits, the phalanges having little motion on each other, 

 though joints remain at elbow and wrist. An increase in the 

 number of phalanges is common, the extreme reached in Glob- 

 iocephalus where the formula is 4, 14, 11, 3.1. This may be 

 the result of actual increase of phalanges or the persistence 

 of epiphyses as separate bones. 

 Ungulata, in foot structure, present two different lines. If fossils be con- 

 sidered, the order must be enlarged to include groups (Hyracoids, Proboscidians 

 and several extinct associations) usually regarded as orders. The fossil groups 

 are scarcely considered here, nor is there an attempt to decide whether modern 

 Ungulates are diphyletic as has been argued. The characters of limb structure 

 of an ancestral group have been summarised by Weber as follows: 



"Humerus with entepicondylar foramen, strong tuberosities and shallow 

 condyles; radius and ulna separate, carpus with centrale; trapezoid and capita- 

 turn (carpalia 2 and 3) small. Femur with third trochanter; tibia and fibula 

 distinct, latter articulating with talus, scarcely or not with calcaneus. Talus 

 with neck and articular head for the naviculare (centrale), its surface for the 

 tibia restricted, behind with an opening (foramen tali). A tibiale tarsi above 

 the ectocuneiforme (tarsale 3). Pentadactyle; ungual phalanges in the older 

 members little broadened. Plantigrade or at most semiplantigrade." 



There are two types of basipodal structure in Ungulates (sens. lat.). The 

 taxeopodous group has proximal and distal tarsal bones in straight lines parallel 



Fig. 319. — Car- 

 pus and metacar- 

 pus of walrus 

 ( M u r i e ) i-r, 

 sea pho-lunatum ; 

 R, radius; U , 

 ulna; u, ulnare; 

 1-5, carpalia; I— V, 

 metacarpals. 



