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Show 44 ZOOL OGY OF' TilE VOYAGE OF THE BEAGLE. .· . ta is broken by a deep round notch immediately ever, the contour of the supcuoi cos 1 . . cess arches in a slight degree . . 1 H' opotamus t 11s pto behind the corac01d: m t 1e tpp 'd d -hallower notch. In the Palceo- .. ndinO' but WI er an s . . backward over a collespo o . sta behind the coracOid, IS as l 'ty of the supenor co , tlteriwn c1·assum t 1e concavi . M . 1 1ia the superior costa of the . . Rl . . , . but ll1 the acmuc lei . hght as m .t he 1.m oce10ts ' t l away f wm t1Ie pm· allel of the spine ' immedwtely scapula begms to nse or s rc c 1 ' 'fi . f tl e spine of the scapula which . 'd The moclt cations o 1 behmd the corac01 process. 1 p 1 d ltave been clearl v and . h R · ts anc ac 1 y erms < ' J characterize respectively t e umman . . · ts out the exceptional £ l b C . who at the same time pom concisely set ort 1 Y uv~er, . 1 . d t' f the acromial angle. condition which the Cameltdce present Ill t le p!Ol uc ~on o 't d all the fragments It was with peculiar interest and care, therefore, t Jat reum e l . t f tl of the sea ula of the Macrauchene, in the hope of gaining from t 11~ par o le skeleton a~ decisive evidence of an affinity to the Camel as the cervtcal vertebrt' had afforded. It unfortunately happens, however, that the .part of .the scapu a most important in this comparison is broken off; yet fro1~ thts ver! cucumstan.ce, combined with a slight inclination forwards of the anteriOr ~argm of the s~me immediately beneath the fractured acromion, and from the .tluckness of the fractured surface, we may infer that the acromial angle of the spme was more pro~ uced than in the ordinary Ruminants, although evidently in a less degree than. m the Camel tribe. The Macrauchenia, however, surpasses these aberrant Rumm~nts, and equals the Pachyderms in the elevation and extent of its sc~pular. spme: but this process commences about half an inch behind the glenoid cavtty, at~d rises at once to the height of three inches above the plane of .the scapula; I.n which structure we may trace the same tendency to the Ru~mant ~y~e, as lS manifested in the scapula of the Hippopotamus and Anoplothenum; form most other Pachyderms the spine increases gradually from its extremities t? t~e middle part. The anterior margin of the spine beneath the short acromwn IS perforated by an elliptical fissure measuring t~n lin.es, Ly three lines. .T~e exte~t of the spine which is preserved, measures e1ght mches and a half; 1t IS a thm and nearly straight plate of bone, expanding into a thick and rugged up~er margin, which slightly over-arches the inferior fossa. (fig. l, Pl. IX.) In Its general form and proportions the spine of the scapula in Macrauchenia presents the nearest resemblance to that of the Hippopotamus; but its origin is closer to the articular surface of the scapula than in this, or any other Pachydermal or Ruminant genus. The portion of the antibrachium of the Macrauchenia which is preserved, presents a condition of the radius and ulna intermediate to those which respectively characterize the same bones in the Pachyderms and Camels. In the former the radius and ulna are separate bones, united in the prone position by ligament, yet so that the movement of supination cannot be performed ; in the FOSSIL MAMMALIA. 4.5 ordinary Ruminants they are partially joined by bony confluence, which rarely extends to the proximal extremities; in the Camel and Llama the anchylosis of the radius and ulna is so complete, that no trace of their original separation can be perceived, and the olecranon appears but as a mere process of the radius. In the Macranchenia the anchylosis of the radius and ulna is also complete, but the boundary line of the two origina11y distinct bones is very manifest, and the proportion which each contributes to the great articulating surface for the distal end of the humerus is readily distinguishable. About a sixth part of this surface is due to the head of the radius, which enters into the composition of the anterior and outer part of the articulation, and its extent is defined by a depressed line describing a pretty regular curve, with the concavity directed forwards and a little outwards. (a, fig. 1, Pl. X.) Just below the articular surface a strong triangular rugged protubemnce projects from the front of the head of the radius, for the attachment of the tendon of the biceps. The line of separation of the radius and ulna is indicated on the inner side of the head of the radius by a deep and narrow fissure extending downwards from below the anterior part of the articula· ting surface; and on the outer side hy a broad groove leading upwards to a deep pit near the proximal end of the antibrachium. We may see by the direction of the head of the radius which is thus defined, that it crosses obliquely in front of the ulna, as in the Elephant, Hippopotamus, and other Pachyderms, and that the bones are anchylosed in the prone condition : below this fissure and groove, which mark the interosseous line, the radius and ulna become blended together into one compact bone, which is flattened from before backwards, with a well marked ridge on the outer side; and excavated by a single medullary cavity, the compact walls of which present a general thickness of one-third of an inch. The proximal articular surface or sigmoid cavity of the ' antibrachium, constituted as above described, resembles that of the Palreothere, Tapir, and the generality of the Pachyderms in having two depressions, instead of three, as in the Anoplothere, and Ruminants. The Hippopotamus has a slight tendency to the latter structure, which is also less marked in the Camel than in the ordinary Ruminants. In its general form the sigmoid cavity of the Macrauchene resembles that of the Hippopotamus more than that of the Camel. In the Camel this articular surface is traversed transversely by a broad, shallow, and slightly roughened tract, which divides the smooth surface of the joint into two parts, one forming the anterior horizontal surface due to the conjoined radius and ulna, the other forming the vertical concave surface on the anterior part of the base of the olecranon. In the Hippopotamus there is, as it were, an attempt at a similar division of the articulating surface at the proximal end of the antibrachia! bones; a deeper and rougher depression encroaches upon the articulation from its outer side, but stops when it has reached half-way across. In the |