PROFESSOR OWEN ON THE MEGATHERIUM. 
753 
thoracic vertebrae of the Ornithorhynchus : the diapophysis, which is single in the 
trunk, not unfrequently becomes double in the tail ; and the hypapophysis commonly 
undergoes the same modification in that region. 
In the Australian Water-rat {Hydromys chrysogaster) there are two pairs of hyp- 
apophyses in the fifth to the tenth caudal vertebrae inclusive, in most of which the 
anterior pair, homologous with that in Helamys, are longer than the posterior pair. 
But in such an exceptional instance it is not necessary to multiply names for the 
superadded or rather subdivided processes ; the purposes of correct and definite de- 
scription being sufficiently answered, by distinguishing under the name ‘ hypapo- 
physis' those, commonly exogenous, inferior processes of the centrum, whether single 
or in pairs, from the autogenous haemal arches and spines, with which they have 
usually been confounded. 
In the anterior thoracic and the cervical region of Crocodiles and Lizards the 
hypapophysis is constant and commonly single, but I have met with fossil vertebrae 
of a Crocodile in which the cervical hypapophysis was divided in the median plane, 
so as to form a pair*. The basioccipital sends down a pair of hypapophyses in the 
Iguana and Mosasaurus^, as likewise in Ruminants. This bifid modification is more 
commonly found in the caudal vertebrae of Reptiles, as it is in Mammals ; but in the 
Iguanodon some of the caudal vertebrae have the hypapophysis single, where, e.g. it 
supports the coalesced bases of the haemapophyses of such vertebrae. 
In the Saw-toothed Seal {Leptonyx serridens) the cervical and anterior dorsal 
vertebrae have a hypapophysial ridge, which, in the latter, is produced into a tubero- 
sity: the lumbar vertebrae are characterized by a pair of hypapophyses from near 
the hinder end of the centrum. 
In the Phoca groenlandica the strong hypapophysial ridge of the lumbar vertebrae 
also divides into two tuberous processes. These processes indicate the great develop- 
ment of the anterior vertebral muscles, e.g. the ‘ longi colli’ and ‘psoae’; and relate 
to the important share which the vertebrae and muscles of the trunk take in the 
locomotion of the Seal-tribe, especially when on dry land, where they may be truly 
called ‘ gasteropods,’ in respect of their peculiar mode of progression. 
The hypapophysis is pretty constantly developed in the cervical region of the 
Ungulate quadrupeds, especially in the artiodactyle division. It is single in the atlas 
of the Hippopotamus, but divides into two ridges, diverging as they extend back- 
wards, and terminating in tuberosities in the second and third cervical vertebrae : in 
the fourth they begin to subside, and disappear in the fifth cervical vertebra. A 
pair of hypapophysial tubercles reappear in the third lumbar vertebra of the Hippo- 
potamus. 
[n the great extinct Irish Deer {Megaceros Hlbernicus) the hypapophysis is a 
* On Fossil Reptiles from New Jersey, Quarterly Journal of the Geological Society, November 1S49, p. 3S0, 
pi. 10. fig. 2. 
t Ib. pi. 10. fig. 5. 
