590 



TITANOTHEBES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Terms used in describing the postcranial sTceleton oj the titanotheres — Continued 



Sacrum, pelvic girdle, hind limb — Continued 



Fibula 



H ead 



Fibulotibial facet 



Shaft 



Fibulocalcaneal facet 



External malleolus 



Calcaneum 



Tuber calcis 



Sustentaculum 



Sustentacular facet 



Ectal facet 



Inferior facet 



Cuboid facet 



Astragalus 



Astragalotibial trochlea 



H ead 



Neck 



Navicular facet 



Cuboidal facet 



Tarsals 



Navicular 



Entocuneiform 



Mesocuneiform 



Ectocuneiform 



Cuboid 



Metatarsals II-IV 



Phalanges 



Fibula 



Caput fibulae 



Facies articularis superior 



Corpus fibulae 



Facies articularis inferior- 

 Malleolus externus 



Calcaneum 



Tuber calcis 



Sustentaculum tali 



Facies sustentacularis 



Facies ectalis 



Facies inferior 



Facies cuboidea 



Astragalus 



Trochlea astragali 



Caput astragali 



Collum astragali 



Facies navicularis 



Facies cuboidalis 



Tarsalia 



Os naviculare 



Os entocuneiforme 



Os mesocuneiforme 



Os ectocuneiforme 



Os cuboideum 



Metatarsalia II-IV 



Phalanges 



fb. 



mal. exi. 

 cal. 

 tu. cal. 



sus. 



ect. 



inf. 



ch. 



as. 



cr. ex., ci 



(nav.) 

 icb.) 



ch. 



ml. II-IV. 



ph. 



SECTION 2. THE POSTCRANIAL SKELETON OF 

 LOWER EOCENE TITANOTHERES 



SUBFAMILY LAMBDOTHEEIINAE , 



Lambdotherium 



Lambdotherium is a small, swift, slender-limbed, 

 relatively abundant animal, less abundant than the 

 contemporary horses but more abundant than the 

 small lophiodonts (Heptodon). The largest Lamb- 

 dotherium measures about 14 inches (350 mm.) at the 

 shoulders, as compared with the smallest Eotitanops 

 (E. gregoryi), which measures about ISJ^ inches (456 

 mm.) at the shoulders. It exceeds in size most of the 

 contemporary species of Eohippus and equals the large 

 EoMppus robusius of the lower Wasatch. 



Lambdotherium popoagicum 



The materials of L. popoagicum are extremely 

 scanty; in only two specimens (Am. Mus. 4880, 14903) 

 are parts of the skeleton associated with the teeth, 

 and these parts are very fragmentary. (See figs. 486- 

 489.) 



In the atlas (fig. 487) the vertebrarterial canal 

 pierces the anterior portion of the base of the trans- 

 verse process; that is the primitive condition in tita- 

 notheres and m Perissodactyla generally. 



The restoration of the fore limb (fig. 488) is highly 

 conjectural, because the lengths of the bones are un- 



known. The proportions are heavier than in Eohippus. 

 The ratio of the radius to the humerus is estimated 

 at 90 per cent, the same as in Eohippus. 



The scapula (fig. 487) presents an elongate neck 

 (tr. 19 mm.) and the lower part of the postspinous fossa 

 is very narrow; the lower part of the prespinous fossa, 

 which is not all preserved, is slightly broader; the 

 spine descends rather low. The distal end of the 

 humerus indicates the presence of a small entocondyle, 

 and its narrow rotula accords with the deep, laterally 

 compressed proximal articular surfaces of the ulna 

 and radius. The length of the radius is estimated at 

 103 millimeters; the shaft is rather narrow and sharply 

 convex. The ulna is elongate and has a high, deep, 

 laterally compressed olecranon process; the proximal 

 portion of the shaft behind the radius is deep. 



The carpus (fig. 489) is correlated with this slender 

 structure, being relatively high and narrow but 

 broader than in Eohippus; the scaphoid rests 

 chiefly on the magnum anteriorly, and the high and 

 narrow lunar exhibits a broad supporting unciform 

 facet and an oblique nonsupporting magnum facet. 

 The magnum, wanting in this specimen, was probably 

 small; it is relatively broad in Eohippus. The cimei- 

 form is rather high and narrow. 



The manus exhibits four digits; the ends of the 



metapodials show a tendency to functional tridac- 



1 tylism, because the median digit (III) is larger than 



