752 
PROFESSOR OWEN ON THE MEGATHERIUM. 
Of the Hypapophy sis. idea of the nature and importance of this process 
may be formed by its constancy and range of variation in the Ophidian Reptiles, 
and especially by its degree of development in the genus Crotalus (Plate II., 
fig. 36-39, hy), which is maintained throughout the vertebrae of the trunk. It is 
present, under various modifications, in certain regions of the vertebral column, in 
most other reptiles ; as likewise in Birds, and in a certain proportion of the mam- 
malian class. 
Goiter seems first to have noticed this process in the Hare, and subsequent com- 
parative anatomists have pointed out this peculiarity of certain lumbar vertebrae of 
the Leporidce, as ‘an inferior spinous process.’ It is not, however, the inferior 
‘ homotype,’ or answerable part on the lower or ventral aspect of the vertebra, to the 
‘spinous process’ above; this is properly repeated by a true inferior or haemal spine 
developed from the apex of the arch on the ventral aspect (Plate LIII., fig. 60, 
h, hs), which arch answers to the neural arch {ib. n,ns) on the dorsal aspect of the 
vertebra. The hypapophysis is a process commonly exogenous and always developed 
from the body of the vertebra, not from the haemal arch. Cuvier applies the same 
term ‘4pine inferieure’ to the exogenous processes from the centrum of the vertebrae 
of the Rattlesnake and Hare, and to the autogenous haemal spines in Fishes. 
In the Hare {Lepus timidus) the hypapophysis appears as a ridge in the posterior 
dorsal vertebrae, is developed into a process in the last dorsal, rapidly increases in 
length in the first and second lumbar vertebrae, inclines forwards in the third, and 
is suddenly reduced again to a mere ridge in the fourth and following lumbars. 
In the Rabbit (Lepus caniculus) it is a low ridge in the posterior dorsals : the ridge 
is produced into a low angle in the first lumbar ; becomes a longer and more slender 
process in the second lumbar ; inclines forwards at a more acute angle in the third 
lumbar than in the Hare ; and subsides to a mere ridge in the succeeding lumbars. 
There is a short hypapophysis from the middle of the inferior part of the ring of the 
atlas, in both the Hare and Rabbit ; and in most other Rodents. In the Guinea-pig 
(Cavia porcellus) a hypapophysis is developed from beneath both atlas and dentata. 
The hypapophysial ridge is strongly marked upon the lumbar vertebrae in the 
genus Dasyprocta : but the most constant location of the inferior exogenous pro- 
cesses from the centrum is in the caudal region, where they are frequently associated 
with a haemal arch and spine. 
In the Cape Hare or Jerboa (Helamys capensis) the hypapophysis, in the middle 
caudal vertebrae, arises from the fore-part of the under surface of the centrum, and 
immediately bifurcates : in some of the vertebrae anterior to these, a haemal arch is 
attached to the ends of the bifurcate hypapophysis : towards the end of the tail the 
hypapophysis appears as two short diverging processes. 
The modification which consists in a single process becoming double, affects most 
of the exogenous processes in certain parts of the vertebral column in the vertebrate 
series. We have already seen that the metapophysis is double in the anterior 
