732 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



From this we deduce the following law, which was 

 partly formulated by the author in 1900 (op. cit., p. 94) : 

 The straightening of the ungulate limb is a secondary 

 adaptation designed to transmit the increasing weight 

 through a vertical shaft and thus relieve the muscles 

 and Hgaments from strain. Correlated with the 

 straightening of the limb are the shifting of the 

 proximal and distal articular facets of the humerus and 

 ■of the femur into direct lines of pressure, and the 

 alteration of the sections of the articular surfaces 

 from an oblique to a right angle, or a horizontal 

 angle with relation to the vertical shaft. 



This law is clearly illustrated in the accompanying 

 figures, based upon full-size vertical sections of the fore 



shaft in Elephas. By these means in the straight- 

 limbed Proboscidea, Dinocerata, and Pyrotheria the 

 proximal articular facet comes to lie almost directly 

 across the top of the shaft, while the distal articular 

 facet similarly lies across the bottom of the shaft. 



Angles. — In the progressive adaptation of the titano- 

 theres to increasing weight we observe a similar modi- 

 fication of the primitive angulation; both fore and hind 

 limbs become more vertical. Even in the middle 

 Eocene forms, such as the subgraviportal Palaeosyops, 

 the hind limb is exceptionally vertical. 



Patella. — The above adaptation is accompanied also 

 by a shifting of the position of the patella. In the 

 primitive, highly flexed ungulate limb the patella is 



Figure 668. — Angulation of the fore limb in graviportal types, as shown by longitudinal 

 sections of the humerus, ulna, and radius 



A, Hhinocnos unicornis; B, Brontotherium leidyi; C, Eobasileus cornuius: D, Elephas indicus. After Osborn. A, B, 

 Incompletely graviportal adaptations, with the humeroradial angle more acute and the proximal and distal 

 articular facets more oblique to the long axis of the shaft; C, D, advanced graviportal adaptations, with the 

 humeroradial angle more open and the proximal and distal articular facets more horizontal to the long axis of 

 the shaft. 



limbs of four types of quadrupeds which exemplify the 

 progressive straightening of the limb, step by step, with 

 progressive increase in bulk and size. (See fig. 668.) 



Axes. — The main axis of each limb bone is regarded 

 as the line a-a, which passes through the center of the 

 proximal facet and behind the center of the distal 

 articular surfaces. It is noteworthy that the edges 

 (6-&) of these proximal and distal articular surfaces 

 are in parallel planes. It will be observed that the 

 angle at the elbow is constantly lessening until in the 

 extreme extension of the elbow joint in the elephant it 

 becomes relatively small. 



Facets. — Another important change takes place: 

 the proximal and distal facets, which are out of the 

 line of the main axis of the shaft in Rliinoceros, are 

 observed to shift more nearly to the extremities of the 



placed at the extremity of the shaft; the patellar facet 

 forms a sharp angle with the axis of the shaft. In the 

 graviportal femur the patella shifts forward to the 

 front of the shaft, and the patellar facet becomes nearly 

 parallel with the long axis of the shaft. 



Head of the femur. — In graviportal animals the head 

 of the femur is sessile upon the top of the shaft, so that 

 the weight of the body, in so far as it is represented by 

 the pressure of the acetabulum upon the head of the 

 femur, is transmitted directly through the shaft of the 

 femur (H. F. Osborn). This arrangement, together 

 with the subvertical position of the ilium, the rela- 

 tively narrow transverse diameter of the pelvis across 

 the acetabula, and the inwardly tilted tibial facet 

 of the femur, brings the feet close to the midline and 

 directly beneath the acetabulum (W. K. Gregory). 



