MECHANICS OF LOCOMOTION 



743 



FORMS OF ILIUM 



In the evolution of the ihum, as in the evolution of 

 the scapula, teleogenetic or analogous adaptation to 

 weight or to speed is far more potent and conspicuous 

 than ancestral influence. Of the ilium, as of the 

 scapula, there are four main types, the primitive (such 

 as that of PJienacodus) , the mediportal {Tapirus), the 

 graviportal {Titanoiherium, Elephas), and the cursorial 

 (NeoMpparion). These four types are fashioned by 

 the demands of the musculature and the movements 

 of the limbs. 



The ilium of titanotheres of the early graviportal 

 type (Manteoceras) and of the advanced graviportal 

 type (Brontops) is analogous to that of the heavy, 



Five types of ilia are illustrated in A, B, C, D, and 

 E of Figure 673 : 



1. The primitive hypothetical ancestral form is shown in A. 



2. The superior border of the ilium of all the early Eocene 

 perissodactyls, so far as known, exhibits the subcursorial form 

 shown in B. 



3. The Eocene titanotheres fall into the incipient or sub- 

 graviportal form E, which represents a considerable advance on 

 the ilium of Tapirus. It is an intermediate form, with greatly 

 broadened ilia, but still exhibits the interrupted superior 

 border &. 



4. The Oligocene titanotheres fall into the extreme gravi- 

 portal form F, with greatly expanded ihac border. This is the 

 extreme weight-bearing type. 



5. The Equidae exhibit an extreme cursorial form (D) in 

 Equus. 



Figure 673. — Seven typical forms of ilia, showing cursorial and graviportal adaptations 



A, Primitive subcursorial (Phenacodus), one-sixtli natural size; B, cursorial (Hyrachyus), one-sixth natural size; C, cursorial 

 (Mesohippm), one-fourth natural size; D, extreme cursorial (Equus), one-eighth natural size; E, subgraviportal (Man- 

 teoceras), one-eighth natural size; F, graviportal (Brontops) , one-t'^eUth natural size; G, graviportal (Teleoceras), one- 

 eighth natural size. 



slow-moving rhinoceroses {Metamynodon, Teleoceras). 

 It presents wide differences from the ilia of extremely 

 cursorial types, such as Hyrachyus, Mesohippus, and 

 Equus. 

 The general contrast is shown below: 



To understand the evolution of the ilium we must 

 distinguish between the crests (cristae) or borders 

 and the fossae areas or surfaces, starting with the primi- 

 tive mammalian ilium (A) and following the progressive 

 changes in the main attachments for the abdominal 

 muscles and the flexors and extensors of the limbs. 

 The actual transitions between this primitive mam- 

 malian (A) of condylarthrous or unguiculate type and 

 the oldest mediportal ungulate type are not known, 

 so that the comparisons made below are somewhat 

 conjectural. 



It is noteworthy that whereas there is a wide con- 

 trast in the scapulae of Tapirus (mediportal) and of 

 Equus (cursorial), the ilia of the same animals are 

 not greatly different in form (fig. 674). 



