i6 Field Museum or Natural History — Zoology, Vol. XL 



the terminal part) are collectively known as mesosternum, while the 

 lower end is called xiphisternum, xiphoid, or ensifomi process of the 

 sternum (see chart, p. ii). The prestemuin is often "keeled," 

 having a ridge along the middle line below, noticeably in the Bats, 

 which like the birds require support for a large pectoral muscle as 

 an aid to flight. 



Scapula, generally a broad, flat bone commonly called the "shoulder 

 blade," which is present in all inaminals. It has a median ridge on 

 the outer surface which is called the "spine," the projecting end of 

 which is known as the acromion process. 



Clavicle or collar bone in Man articulates with the upper border of the 

 sternum and the acromion process of the scapula. It is present 

 and complete in soine mammals, such as Primates (Man, Monkeys, 

 Apes, etc.). Bats, Insectivores, and others; and appears in rudi- 

 mentary form in others, such as most of the Carnivores, some 

 Rodents, etc., but it is absent in Whales, Seals, Ungulates, some of 

 the Bears, some Rodents, and others. 



The Limbs and Feet — All mammals, with the exception of some of the 

 aquatic species such as Whales, Manatees, etc., have four limbs and 

 for this reason are often called Quadrupeds. In the fore limbs the 

 bone of the upper arm is called the humerus; the lower part of the 

 arm or "fore arm" has two bones, the radius and idna. The wrist 

 or carpal joint consists of several bones, usually 5 to 8, known as the 

 bones of the carpus. The bones of the hand are designated as 

 metacarpals and those of the fingers, phalanges. 



As has already been stated, the hind limbs are lacking in aquatic 

 species, such as the Whales and Manatees. In the former even the 

 pelvis is rudimentary and is represented by two small detached 

 bones not connected with the skeleton. The bones of the hind 

 limbs are: Thigh, femur; low^er leg bones, tibia and fibula; ankle joint, 

 bones of the tarsus; bones of the foot, metatarsals; and toes, phalanges. 

 The variation in the character and shape of the limbs and feet 

 is very great, as illustrated by the "flippers" of a Seal, the feet of a 

 Horse or those of a Lion, Deer, Sloth, Monkey, Mole, etc. In 

 some species practically the whole sole of the foot touches the 

 ground in walking; these are known as plantigrade. Others walk 

 on their toes and are called digitigrade.* In the Horse, for example, 

 the true heel is elevated a foot or more from the ground. 



In nearly all mammals f the terminal extremities of the digits are 

 protected by hard epidermal structures in the form of claws, nails, and 



* Intermediate types are often termed semi-plantigrade, iinguligrade, etc. 

 t Absent in the Whales, but rudiments have been found in the foetus. 



