372 
PROFESSOR OWEN ON THE MEGATHERIUM. 
physes. The inferior or heeiiial arch is also augmented by increased length of the 
ribs. The key-bone of that arch, sterneber or haemal spine, Plate XXVII. figs. 4-7, 
repeats the characters of that of the previous segment, save that the sternal articula- 
tion, s, and the hsemapophysial ones, hp, hp, of the central division of the bone are 
more continuous, as shown in fig. 6. Fig. 5 shows the surface which was presented 
towards the integument ; fig. 4 that which was turned towards the cavity of the chest ; 
fig. 7 is a side view showing the four articular cavities for the double condyles of the 
sixth and seventh haemapophyses ; fig. 6 shows the under surface of this remarkable 
type of sternal bone. 
In the seventh dorsal vertebra (Plate XIX. figs. 1, 2, 3), a third articular surface, 
mz, fig. 1, is developed between the two anterior zygapophyses, z, z, to join that upon 
the back part of the sixth vertebra; so that there are three zygapophyses, a median 
and two lateral, on both the fore and the back part {mz', fig. 2) of the arch of this 
vertebra, making, with the three articular surfaces on each side (fig. 3, c, n', d') for 
the ribs, and with the anterior and posterior surfaces of the centrum, not fewer than 
fourteen joints. In this and the two following segments of the back (D s and 9, 
Plate XVII.), the ribs attain their greatest length. In the tenth segment the heem- 
apophyses cease to articulate below directly with a heenial spine. The median zyg- 
apophyses continue to be developed both before and behind to the twelfth dorsal ver- 
tebra inclusive. In the thirteenth (Plate XXVI. fig. 4) this supplementary articula- 
tion is suppressed behind ; and the costal articulations have disappeared from the 
diapophyses d. Those on the neurapophyses are almost circular, n', and those on the 
upper and posterior angles of the centrum, c', have increased in size. The metapo- 
physis (m), which was indicated by a protuberance above the diapophysis in the pre- 
ceding dorsals, begins to assume the form of a rugged thick vertical ridge. The 
fourteenth dorsal vertebra shows the progressive increase of size of the centrum, and 
the absence of the median zygapophysis before as well as behind ; and in it the 
costal articulation on the centrum for the penultimate rib is lost, as well as that on 
the diapophysis for the antepenultimate one, and only the subcircular concave neural 
surface for the rib remains. In the fifteenth dorsal vertebra the posterior zygapo- 
physes are convex transversely at their inner border, slightly concave in the rest of 
their extent; the back part of the neural spine between these processes is deeply 
grooved ; the rnetapophysial ridge increases in height and length ; a short and thick 
anapophysis is developed from the back part of the base of the diapophysis, and on 
the under part of the anapophysis there is a distinct, nearly flat, articular surface. 
The sixteenth dorsal vertebra (Plate XIX. figs. 4 and 5, Plate XXVI. fig. 5) offers a 
corresponding modification at the fore-part of each neurapophysis, in the develop- 
ment of a short, strong, wedge-shaped process, p, fig. 5, answering to the parapo- 
physis in Myrmecophaga* and Dasypus, with an articular surface, pa (Plate XIX. 
fig. 4), on its upper part for junction with the anapophysis of the preceding vertebra. 
* Philosophical Transactions, 1851, Plate L. fig. 22, p, pa. 
