32 COMPARATIVE ANATOMY. 



Mammalia. Scapula and coracoid bones united, excepting 

 Monotremata ; clavicle absent in ungulate quadrupeds, as in 

 Bos (bull), present in unguiculates, as in Sdurus (squirrel). 

 In ungulates the ulna and fibula are often rudimentary and 

 anchylosed to sjiaft of radius or tibia. Toes, never more 

 than five to each extremity, enormously developed in length 

 in anterior extremity of Cheiroptera (bat) ; joined with tar- 

 sus and metatarsus in presenting an opposing surface for 

 progression, in which case all five toes are present (planti- 

 grade unguiculate), as in Ursus (bear) ; tarsus and metatar- 

 sus elevated, and an opposing surface obtained on tips of 

 phalanges, in which case a number less than five is present, 

 the first toe commonly rudimental or absent (digitigrade un- 

 guiculate), as in Canis (dog). Ungulates are properly digiti- 

 grade, and, excepting Mephas (elephant), have less than five 

 toes, as follows, no mention being made of the absent digits : 

 Sus (hog) has third and fourth toes, with hoofed rudiments of 

 second and fifth; Rhinoceros, second, third, and fifth; Bos 

 (bull), third and fourth, with concealed rudiments of second 

 and fifth metatarsal; and Equus (horse), third, with con- 

 cealed rudiments of second and fourth metatarsal (splint 

 bones).* The elephant is thus seen to have five toes to each 

 foot, the rhinoceros three, the hog and bull two, and the 

 horse but one. 



In this descent from five to a smaller number, the toes, 

 whether of a plantigrade or digitigrade foot, are always re- 

 moved from the sides, and are followed by loss of corre- 

 sponding metatarsal and tarsal elements. But whatever their 

 number and degree of development, the relations presented 

 between the parts of a foot are constant; so that in the event 

 of absence of all the toes excepting the third, the relations 

 of this single toe to the bones above it are the same as though 

 all five toes were present, f 



* The phalanges of the foot of the horse are called, from above down- 

 ward, the pastern, coronary, and coffin bones. 



f A vertebrate foot may be defined to be that assemblage of bones placed 

 toward the distal portion of the posterior extremity beyond the tibia. In 



