PROFESSOR OWEN ON THE JMEGATHERIUM. 
755 
the anterior, and d! d! the posterior diapophyses. These are, also, shown in the view 
of the under surface of the same vertebra, fig. 62, b, in which the hypapophyses hy, 
hy are figured. Fig 62, c, gives an anterior view of the centrum with mm the 
metapophyses, dd the anterior diapophyses, and hy the hypapophyses, diverging from 
it, as exogenous processes. The neurapophyses n n and the posterior diapophyses d! d! 
are also seen. 
Many excellent figures of the caudal vertebrae of different species of Mammalia 
are given in the ‘ Osteographie ’ of M. De Blainville, the characters of which may 
be readily understood by the above definition of the processes that diverge from them. 
In the class Aves, the hypapophyses attain their maximum of development and 
range of variation in the Penguins {Aytenodytes patachonica, Plate LIL, fig. 48). 
Single and long on the first four vertebrae of the neck, especially on the dentata, 
their place is taken by a pair of short processes in the sixth cervical, which pro- 
cesses increase in size in the four following vertebrae, converging in the last {Jih, c lo) 
so as almost to complete a bony canal for the carotids: in the eleventh cervical (c u) 
they have coalesced again into a single median plate, with its extremity extended 
both forwards and backwards ; this plate decreases in the three following vertebrae. 
In the next, which may be regarded as the first dorsal, the hypapophysis again 
appears as a pair of processes (d i , hh) \ both of these are broad plates, almost coex- 
tensive with the under surface of the vertebra and diverging from each other (fig. 
49, hy). The common base of the diverging plates progressively lengthens in the 
second (fig. 50) and third dorsals, and the hypapophysis appears in the succeeding 
ones as a strong vertical plate, simply expanded at its end, and growing somewhat 
shorter to the last (fig. 51, n 4 , hy). 
In the Sphceniscus minor, the first dorsal vertebra, or the fourteenth vertebra from 
the head, presents three hypapophyses, which appear to be blended together in the 
next vertebra so as to form the common base of a pair of diverging plates. The 
hypapophyses in the Penguins have an analogous function to that in the Seals, 
extending the surface of attachment of the powerful muscles on the ventral aspect 
of the vertebral column, which so materially aid in the shuffling gasteropodal move- 
ments of both kinds of aquatic animals, when on land. 
The hypapophysis is well-marked in the anterior cervical vertebrae, especially the 
dentata, in most birds : it usually reappears, as a pair of processes, from the par- 
apophyses of the lower cervicals ; which hypapophyses converge as they approach the 
dorsal region, and coalesce into a single process in the anterior vertebrae of this 
region, and in some birds in the lower cervicals. 
In the Pelican, the parial hypapophyses coalesce at their lower extremities, and 
form a haemal arch for the carotids in many of the lower cervicals. 
The anterior trunk -vertebrae in many Siluroids are anchylosed by a liypapophysial 
development of bone continued from beneath ona vertebra to another, and which is 
sometimes perforated lengthwise by a canal lodging the aorta*. The neck of the 
*' On the Archetype of the Vertebrate Skeleton, p. 92. 
