EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



677 



As has often been stated, the manus shows some distinctly 

 artiodactyl features, the naost notable being the retention of 

 four digits with tlie main axis between digits 3 and 4, rather 

 than lying in the third itself. Another remarkable feature is 

 the extreme flexibility of the carpus, esiseciaUy in the develop- 

 ment of a true ginglymoid joint between the proximal and 

 distal row of carpals. * * * AH of the elements are present 

 in the carpus, with the exception of the trapezium, of which 

 the last vestige has disappeared. The proximal facets are 

 shown in Figure 1 of Plate IV, though the limits of the radial 

 and ulnar areas are not with certainty definable. 



The scaphoid articulates with the lunar by two facets sepa- 

 rated from each other by a roughened trough. The superior 

 scapholunar facet is long and narrow, its short axis vertical 

 and straight, while its longer axis sweeps to the rear in a gentle 

 convexity. It lias the same anteroposterior extent as the 

 scaphoradial facet above. The inferior scapholunar facet 

 is much smaller, having but half the fore-and-aft extent of the 

 superior. Distally the scaphoid articulates with the trapezoid 

 and the magnum and together with the lunar forms the deep 

 groove into which the pulley-like pivot of the magnum fits. 



The lunar is a somewhat larger bone than the scaphoid, 

 articulating distally both with the magnum and the unciform. 

 The articulation between the lunar and cuneiform is again 

 double, the two facets being separated by a well-defined chan- 

 nel, which runs backward and slightly upward. The two 

 lunar-cuneiform facets are about equal in area. One can form 

 a very good idea of the distal lunar facets by the study of their 

 complementary facets figured in Plate IV, Figure 2. 



The cuneiform is about half the bulk of the lunar and pre- 

 sents two facets on its inner face in every way the complements 

 of the lunar-cuneiform. On the pro.ximal face there is a 

 large, saddle-shaped facet for the ulna, and a smaller, semi- 

 circular cuneiform-pisiform facet in the rear, set almost at 

 right angles with the plane of the first. Distally there is a 

 large cuneiform-unciform facet, having the general form of an 

 equilateral triangle with rounded angles. It is again saddle- 

 shaped, concave in its fore-and-aft axis. 



The pisiform is lacking from the right manus, but that of 

 the left is present and well preserved. It is much compressed 

 laterally, with a deep vertical expansion of the distal end, which 

 is decidedly rugose. The bone presents a gentle, sweeping 

 curve through an arc of nearly 90°. Proximally it bears two 

 well-defined contiguous facets for articulation with the cunei- 

 form and ulna respectively. 



Of the distal row of carpals the trapezoid is absent, having 

 been replaced in the mount by that from another individual. 

 It is not a precise fit, there being some variation between its 

 facets and those of the original bone. 



The articular faces are well shown in the figure, and it 

 will be noted that lateral movement is impossible, while a 

 remarkable range of flexion is indicated. 



The magnu7n has on its lower face facets for the articulation 

 of metacarpals 2 and 3, that for 2 being rectangular, about 

 four times as long as wide. The pivot of the magnum is high 

 and prominent, as indicated in the figure. 



The unciform is the largest bone in the carpus, with the 

 possible exception of the lunar. Distally it bears two facets 

 for metacarpals 4 and 5, while on the radial side there is one 

 which articulates both with the magnum and with meta- 

 carpal 3, the limits of the two articulations not being discernible. 



A study of the distal carpal facets and the proximal meta- 

 carpal ones gives evidence again of more or less fore-and-aft 

 movement, but in the case of the median metacarpal no lateral 

 movement at all. The lateral metacarpals, on the contrary, 

 were capable of lateral as well as fore-and-aft movement, so 

 that, while the foot would spread somewhat when the creature's 

 weight was borne upon it, it was all in the lateral bones. 



This would seem to be stiU further evidence that the true axis 



of the foot was between digits 3 and 4, as in the artiodactyls. 



The principal dimensions of the manus are: 



Millimeters 



Width of proximal facets 170 



Width of distal carpals 170 



Depth, lunar to summit of metacarpal 3 80 



Length of metacarpal 3 250 



The hind limb 



The entire limb is figured in Plate IV, Figure 4. There was no 

 trace of the pelvis found associated with No. 327, though the 

 limbs are in excellent preservation and give but little evidence 

 of distortion by crushing. 



The femur. — This is a fine bone, notable for its extreme flat- 

 ness, which indicates the pillar-like posture of the bone, as in 

 the elephant, as the shaft would not have been sufficiently 

 rigid to withstand springing had the thigh been flexed. 

 Another interesting feature is the absence of a third trochanter — ■ 

 a character given by Marsh in his definition of the genus 

 Megacerops. There is a ridge on the outer side of the femur 



Figure 611. — Mounted skeleton of Bron- 

 tops brachycephalus? 



Oblique front view. In Victoria Memorial Museum, 

 Ottawa (shown also in fig. 610). About one thirty- 

 fifth natural size. 



continuous above with the great trochanter, which probably 

 represents the vestige of the third. The measurements are: 



Millimeters 



Length 785 



Width of proximal end 236 



Width of distal end 204 



Width of mid shaft 117 



Depth of mid shaft 60 



The tibia. — The general form of this bone is well shown in 

 the figure and calls for no special comment. The measure- 

 ments of the tibia are: 



Millimeters 



Length 446 



Width of proximal end 200 



Depth of proximal end 132 



Width of mid shaft 80 



Depth of mid shaft 77 



The fibula is quite slender with expanded articular extremi- 

 ties; length, 395 millimeters. 



The pes. — The general proportions are in keeping with those 

 of the manus. All of the tarsal elements are represented, with 

 the exception of the entocuneiform, which is entirely lacking. 



