328 AN INTRODUCTION TO THE STUDY OF FOSSILS 



arranged in five toes (Fig. 141, A). The hind limb is composed 

 of the hip bone or innominate, firmly attached to the sacral 

 vertebrae, and made up of the ilium, ischium and os pubis, the 

 thigh-bone (femur) with the knee-cap (patella) protecting its 

 lower edge, the bones of the foreleg (tibia and fibula), the seven 

 ankle bones (tarsals) arranged in two rows, five metatarsals 

 and twelve phalanges (three to each of the four toes) (Fig. 

 141, B). 



The clavicle is vestigial in the Carnivora and entirely wanting 

 in many of the Ungulata. In those vertebrates, however, in 

 which the fore Umbs are capable of a great variety of motions 

 and a freedom of movement, as in flying birds, bats and in pri- 

 mates, it is well developed. 



Hind limbs are wanting among the Sirenia and Cetacea, though 

 a small, degenerate, functionless hip bone is present. In 

 Halitherium, a Miocene sirenian, a vestigial femur is present. 

 In the snakes both fore and hind limbs are absent ; but a few 

 forms, as Python, have a vestigial hip bone. 



The primitive number of digits to each foot is five, but a re- 

 duction occurs in many animals. In the birds there are usually 

 but three upon the wing and four upon the foot ; in many reptiles 

 the number is reduced to three ; among the mammals the pig 

 has four digits, the rhinoceros three, the camel two and the 

 horse but one. 



The source of the foot prints made by the pads and claws of the 

 cat's feet in walking upon a yielding surface, such as dense 

 mud, is rather easily recognizable ; so, too, that of other living 

 animals. By being covered with sediment these footprints 

 may be preserved fossil. Many such vertebrate fossils occur 

 from the Mississippian to the present. The Triassic sandstone 

 of the Connecticut River valley contains especially notable 

 examples. These footprints made upon an ancient mud-flat 

 are mostly due to the reptiles of the time. 



The bones of the fore limbs are attached to the thin triangu- 

 lar scapula, the head of the humerus fitting into a socket in the 

 lower end of this bone, while the hind limbs are similarly 

 attached to the innominate, the head of the femur fitting into 

 a socket in this bone. Both are thus ball-and-socket joints, 



