PROFESSOR OWEN ON THE MEGATHERIUM. 
731 
apophysis is suddenly shortened: the metapophysis and anapophysis are distinct in 
the eleventh dorsal, and so continue to the penultimate lumbar. 
In the Hydromys chrysogaster, with d 14, l^J, the common tubercle divides on the 
twelfth dorsal, and the met- and an-apophyses are distinct on the five following- 
vertebree. 
In the Rat {Mus rattus), with d 13, / 6, the common tubercle, which first appears 
on the sixth dorsal, divides into metapophysis and anapophysis in the ninth. The 
anapophyses become obsolete in the last two lumbar vertebrae. 
In the Cape-Jerboa {Helamys capensis), with d 12, 17, the diapophyses of the first 
dorsal vertebra are unusually long and strong: the anapophysis begins to be de- 
veloped from the back part of the diapophysis of the eighth dorsal, and the metapo- 
physis from the front part of that of the ninth ; in the tenth dorsal (Plate XLVI., 
fig. 9) both metapophysis {m) and anapophysis {a) are distinct from the diapophysis 
{d). In the twelfth dorsal the metapophysis ascends upon the prozygapophysis, the 
anapophysis has increased in length, and the diapophysis is retained. Both ana- 
and met-apophyses are of considerable length in the lumbar region, except in the 
two last vertebrae. 
The progressive development of the accessory tubercle, and its transformation into 
metapophysis and anapophysis, are well exemplified in the last five dorsal vertebrae of 
the Beaver {Castor fiber). 
In the Porcupine {Hystrix cristata), with d 15, I A, the accessory tubercle begins 
to be developed upon the fourth dorsal, and, progressively increasing, it divides in 
the eleventh into metapophysis and anapophysis. The distinction of these from the 
diapophysis which supports the last floating rib is well-marked in the last dorsal 
vertebra. In the lumbar vertebrae the anapophyses strengthen the joints by under- 
lapping the metapophyses, but both processes become rudimentary in the last lumbar 
vertebra. 
In the Coypu {Myopotamus Coypus), with did, / 6, the common tubercle appears 
upon the sixth dorsal ; progressively expands, and in the twelfth divides into met- 
apophysis and anapophysis: the latter subsides in the last lumbar vertebra, but the 
metapophysis is continued along the sacral and a great part of the caudal region, 
continuing long after the true zygapophyses have disappeared. 
In the Paca {Coelogenys Paca), with d 13, 1 6, the common rudiment of the met- 
and an-apophyses extends above the diapophysis of the seventh dorsal as a broad 
ridge ; in, the eighth the ridge is bilobed, in the ninth the lobes diverge ; in the tenth 
and eleventh they become distinct processes : the anapophysis disappears in the last 
lumbar, but the metapophysis is retained in all the lumbars. 
In the Hare {Lepus timidus), with d\2,l7, both metapophysis and anapophysis 
appear abruptly on the ninth dorsal : the metapophyses progressively increase to the 
second lumbar, and are continued, remarkably developed, throughout that region : 
the anapophysis becomes a low long ridge in the last dorsal, and continues, as such. 
