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XXXI. On the Megatherium {Megatherium Americanum, Blumenbach). 
By Professor Owen, F.R.S. ^c. 
Part I. — Preliminary Observations on the Exogenous Processes of Vertebrae. 
Received November 8, 1850, — Read January 9, 1851. 
Before entering upon the description of the skeleton of the Megatherium, it is 
requisite to premise some remarks on the vertebrae of the Mammalia in general. 
Hitherto these parts have been described by means of the terms supplied by Human 
Anatomy. But the skeleton of Man is one which deviates most from the common 
archetype : some parts are developed in excess ; other parts, which are present as a 
general rule in the Mammalian series, are either rudimental or absent. The latter is 
more particularly the case with those processes of the vertebrae which are developed 
in relation to the attachment and force of the muscles ; and which, being connected 
with particular modes and media of motion, become eminently significative of 
the habits and affinities of the species. The consideration and comparison of these 
processes, therefore, most of which have received no names in Human Anatomy, are 
essential to a right determination of the habits and affinities of the Megatherium ; 
and in order to make intelligible the terms in which the vertebral peculiarities of that 
great extinct animal will be described, I propose first to illustrate them by giving a 
comparative survey of the principal modifications of the exogenous processes in the 
vertebrate series. 
The ‘exogenous’ processes of a vertebra are those which grow out of the pre- 
viously ossified parts, and are so classified and named in contradistinction from the 
‘autogenous’ parts or elements of a vertebra which are developed from their own 
proper centres of ossification^. 
In the sixth cervical vertebra, for example, of the human foetus, the part which 
SoEMMERRiNG, the most exact and classical author on Anthropotomy, calls “ radix 
prior seu antica processus transversi vertebrae -I-,” is developed from a separate centre 
and for some time continues to be a distinct bar of bone ; whilst the part called 
“radix postica processus transversi vertebrae^,” grows out of the base of the “radix 
arcus posterioris as ISoemmerring denominates the autogenous part which I have 
called ‘ neurapophysis :’ and from this element, or from the arch formed by its 
coalesence with its fellow, when it constitutes the “arcus posterior vertebrae” of 
SoEMMERRiNG, Other processes grow out; as, for example, the “processus obliqui seu 
* On the Plesiosaurus macrocephalus , Geological Transactions, vol. v. 2nd series, p. 518, 1838. 
t De Corporis Humani Fabrica, 8vo. 1794, tom. i. pp. 239, 241. I Ib. § lb. p. 236. 
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