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Show 46 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. · 1 . ·hened surface ( b · fi g. 1 ' Pl. X.) commencing also at· th· e Macrauchema t 1e wug ' . ss the articular surface : 1t IS, outside, extends only one-third of theTwl ay at~Io lar surface on the anterior part 1 ]1 . the Camel le ar tcu ' 1 however, as s Ja ow as m ·d . th Hil)popotamus than in the Ccune ; f l l · 1on is broa er m e · of the base o t le o ccrar . d . tlle Hippopotamus. The Size · t · t e as broa as m but in the Macrauchene I lS WIC d tl t of the HipJJOpotamus, and a · h M uchene excee s Ia of the olecranon Hl t e acra . . 1 agnitude the Macraucbenia fortiori that of the Camel: indeed m Its genera mcb t 't no doubt had a more c. 11 11 d the largest Hippopotamus; u I must have w Y equa e • k Th lecr·anon of the Macraucbcnia 1 b d d bulky tnm · e 0 shapely, and ess roa an f l C 1 and Hippopotamus; it terminates differs in shape, both from th~t o t le ~me •. and resents a well marked proabove in a three-sided cone With an obtuse. ape~, . p . t . n either the . d f tl e base whtch IS not pi esen l tuberance at the outer SI e 0 • · 1 . ' • . 1 :d ·e on the back Camel or Hippopotamus. There IS also a strong .m~gec . ~~o gthe level of the part of the olecranon which makes an angle before smkmg m ulna below. 1 · t tl Pacbyder The confirmation of the close affinity of the Macrauc lema 0 ·. 1e 1 1 - matous Order, which the structure o f t I1 e cervi·C a 1 ve rtebr.m alo.n e mtcbr 1t Plla veX rIe n) -. dered very doubtful, is afforded by the bones of the nght f~re-foot _( 1 · . 1 · . ' these are fortunately in so penc.e ct a con dt' fw n, as to make It certam t mt t ·lu s interesting quadruped had three toes on the fore-feet, and not mo~e; andbth~lt t_ 1 1 e fully develOJJed metacarpal bones are cl 1. st·m c t , an d COl Tespond m n.u m e1 wit 1 the toes and are not anchyloscd m· to a sm· g1 e c·a nn on bone ' as m the Ru-l minants.' The bones preserved are the metacarpals, proximal phal.ano·e~, anc middle phalanges of each of the three toes, and the distal phalanx of the mner-most toe. 1 · 1 The proximal end of the innermost metacarpal bone presents t uee arucu ar surfaces; the middle facet is the largest, and the two lateral o~es slope aw~y from it at an angle of 45°. The middle facet is broad and shghtly conve~ m front, narrow and coneave behind; the distal articular surface of the trapezotdes must have corresponded with this surface ; the outer facet is narrow, flat, extends from the fore to the back part of the head of the bone, and must have. been adapted to a correspond·m g surface on t 11 e os magnum,· th e H· 1n er facet IS the smallest, presents a tn·a ngular form, an d I·S st· tuaten·1 t owar· d s tl1e bac. k part of the head of the metacarpal bone; it indicates the existence of a rudunental n~cta~ carpal bone, or vestige of a pollex. Below the outermost of the lateral surfaces there 1. s a crescenti.c arti. cular surface w·t t1 1 I· ts concav1' t y d'I r ected outwards and downwards (fig. 2, Pl. XV.), against which a corresponding convex articular surface of the middle metacarpal abuts, (fig. 3, Pl. XV.) External to this surface the proximal end of tl1e middle metacarpal bone presents two articular ~urfac~s for the carpus ; the larger one, whi.C h was adapted to t h e os mag num • IS bon- FOSSIL MAMMALIA. 47 zontal, broad and convex before, narrow and concave behind; the outermost facet is a small triangular surface inclined downwards to the level of the articulating surface of the outermost metacarpal. It also presents a posterior vertical articular surface for a sesamoid bone. The proximal extremity of the outer metacarpal bone is joined to the middle metacarpal, not by one semilunar surface, but by two separate articulations of small size (fig. 4 and 5, Pl. XV.); it presents a single large slightly convex articular surface for the os magnum, of an irregular semicircular form, with the convexity of the curve turned outwards. The metacarpus increases in breadth as it approaches the phalanges ; the two lateral metacarpals bending slightly away from the middle one, and expanding towards their distal extremities : the middle bone presents a symmetrical figure except at its proximal extremity (fig. 2,. Pl. XI.) The distal articulating facet of each of the metacarpal bones extends so far upon both the anterior and posterior surfaces as to describe more than a semi-circle (fig. 3, Pl. XI.); in the two lateral metacarpals it is traversed throughout by a longitudinal convex ridge dividing it into two equal lateral parts ; the ridge is most produced on the posterior half of the joint (fig. 4, Pl. XI.): in the middle metacarpal this ridge subsides before it reaches the anterior part of the articular surface. The proximal extremity of the middle proximal phalanx presents a posterior notch corresponding to the above partially developed ridge: the proximal extremities of the lateral phalanges are traversed by a middle longitudinal depression, and two lateral shallow concavities (fig. 6, Pl. XI. ;) but these are of such an extent as to be in contact with only a part of the convexity above, which therefore was doubtless adapted to a sesamoid bone on each side of the longitudinal ridge. The structure of the above described joints proves that the motion of the toe upon the metacarpus was much freer and more extensive than in the Rhinoceros, which is the only existing Ungulate mammal which presents the tridactyle structure in the fore-foot. In this species the metacarpo-phalangeal articulations exhibit only a slight trace of the longitudinal ridges and grooves which are confined to the posterior part of the joint; these are more developed in the Cam.elidte; but the Hog and Horse in this respect approach nearer to the Macrauchene, though the structure of the metacarpo-phalangeal joints in the Hog falls far short of the compactness and strength combined with freedom of play in flexion and extension which distinguish those of the lVIacrauchene. The Palceotlterimn medium most resembles the Macrauchene in the structure of the trochlear metacarpo-phalangeal joints; but both in this species,* and the Pal. cmssum-j' the articular surface at the distal end of the metacarpal is relatively narrower than in the lVIacrauchenia; moreover all the species of the extinct Palmothere differ from the Macrau- • Sco Osscm. Fossiles, Pl. XX. fig. 3. t Loc. cit. Pl. XXII. fig. 6. |