294 



VERTEBRATE SKELETON 



calcaneus often projects backwards for the insertion of the tendon of 

 Achilles. 



The other tarsal bones fuse in various ways, often accompanied by- 

 reduction of digits and metacarpals. The centrale, when distinct, 

 is the scaphoid (navicular) of human anatomy, the tarsalia have 

 specific names which were given on p. 277. The third digit is 

 usually the longest, but Artiodactyls have 3 and 4 equal and i and 5 

 are largest in Pinnipeds where they support the margin of the flipper. 

 The hallux is opposible to the other digits in Primates and a few 

 members of other orders. In mammals with great leaping powers 

 metatarsals 3 and 4, fuse to a long 'cannon bone,' and the same 

 name is given to the third metatarsal of Perissodactyls. In the 

 former group metatarsals 2 and 5 are reduced 

 to 'splint bones;' in Perissodactyls metatarsals 

 2 and 4 have this name, all being short rudi- 

 ments at the proximal end of the cannon bone, 

 with which they may fuse. 



MoNOTREMES have an entepicondylar foramen in 

 the humerus; radiale and intermedium form a scapho- 

 lunatum, and all feet are plantigrade pentadactyle. 

 The fibula does not form a malleolus, although it articu- 

 lates with both talus and calcaneus. 



Marsupialia usually have the entepicondylar fora- 

 men; radius and ulna can be slightly rotated, radiale 

 and intermedium are usually separate, the latter small. 

 A separate centrale does not occur in the adult. The 

 five digits of the manus are normal, except in Perameles 

 where 4 is rudimentary, i and 5 are lost and the meta- 

 carpals are long. The femur lacks the third trochanter; 

 tibia and fibula have slight motion on each other in 

 arboreal species, but their distal ends are often fused 

 in other species. The digits are variously modified; 

 in kangaroos i is lost, 2 and 3 are reduced and 4 is 

 longest and strongest. 

 Insectivora. — The humerus has an entepicondylar foramen; this bone being 

 peculiar in shape in Talpids. All five digits of the manus are usually present, the 

 poUex never being opposible. Clirysochloris is noteworthy for having the 

 phalangeal formula 2, 2, i, 3, and for having only carpale 3 and the hamatum in 

 the carpus. Other genera often have digit 3 elongate and a separate centrale. 

 The fossorial species have the prepoUex developed as a large falciform bone 

 (fig. 317). A third trochanter on the femur is all but unknown; tibia and fibula 

 are fused distally. The tarsus is sometimes elongate; digit i is lacking in some. 

 Chrysochloris has two phalanges in each toe. 



Fig. 317. — Foot of 

 Talpa (Flower, '85). /, 

 falciform bone; j, inter- 

 inedium; R, radius; r, 

 radiale; U, ulna; n, ul- 

 nare; 1-5, digits. 



