748 
PROFESSOR OWEN ON THE MEGATHERIUM. 
swells outward from its confluence with the centrum, and developes from each angle 
a transversely elongated zygapophysis ; that (z) from the anterior angle looking 
upwards, that (z') from the posterior angle looking downwards, both surfaces being 
flat and almost horizontal. A wedge-shaped process, which I have termed ‘ zygo- 
sphene*’ (fig. 25, 2 :^), is developed from the fore-part of the base of the spine; the 
apex of the wedge being truncate or emarginate, and the converging sides presenting 
two smooth, flat, articular surfaces. This wedge is received into a cavity, which 
I have called ‘ zygantrurn-f'’ (fig. 26, 2 a), excavated in the posterior expansion of the 
neural arch, and having two smooth articular surfaces to which the zygosphenal sur- 
faces are adapted. Thus the vertebrae of Serpents articulate with each other by eight 
joints, in addition to those of the cup and ball on the centrum ; and interlock by 
parts reciprocally receiving and penetrating one another. This is the most con- 
spicuous, but is not, as has been affirmed the peculiar characteristic of the Ophi- 
dian vertebra; the zygosphenal (zs) and the zygantral (za) surfaces being developed 
in certain Lizards, as e. g. the Iguana (Plate LI., figs. 40-42, 2 a, 2 .s) ; where the 
zygosphene ( 2 .?, fig. 41) is deeply notched anteriorly, and the zygantra (za, fig. 42) 
are shallow and separated from each other behind. 
In the vertebree of Reptiles and Mammals with double ' tenon-and-mortise’ joints, 
the only articulating processes which are truly homologous are the zygapophyses 
( 2 , 2 ' in all the figures) : the superadded processes in the Reptiles are, the zygosphene 
( 2 .?) and zygantra (za). A median ‘ tenon’ and two lateral ^ mortises’ at the fore- 
part of the vertebra, and a median ‘mortise’ and two lateral ‘ tenons’ at the back 
part of the vertebra, all of them above the true zygapophyses, are the result of the 
superaddition in Serpents ; whilst in the Edentates a median mortise and two lateral 
tenons in front, and a median tenon and two lateral mortises behind the vertebra, 
are all of them below the true zygapophyses, and are the result of the additional 
processes called ‘ metapophyses ’ and ‘anapophyses ;’ whilst a second pair of ‘tenons 
sustaining transverse processes in the Lacertians and Ophidians, which is indicated by the term ‘ diapophysis ’ 
applied to those processes. If, however, the lower articulating process in the bifurcated anterior ribs of the 
Crocodile be not regarded as the superadded part, but as answering to the head of the simple rib in the Snake, 
then the transverse process to which this is attached will be a ‘ parapophysis,’ and the position of such trans- 
verse process in certain parts of the vertebral column, would lead to that supposition. But the modifications 
of the processes in question, in the Ophidian series, would indicate them to include, as in the BatracMa and in 
the neck of the Plesiosaurus, both diapophysis and parapophysis : and it is in this extended sense that I apply, 
for the sake of convenience, the term ‘ diapophysis’ to the entire process in Ophidia and Sauria. 
* Zuyoj', a yoke, oipiiv, a wedge, 
t Tjvyhv, and avrpov, a cavity. 
I Cuvier, Leqons d’Anat. Comp. Ed. 1835, t. i. p. 216; where, after the double articular processes are 
described, it is stated, ‘‘ Ces apophyses sont agencies de maniere qu’il resulte, comme pour les vertebres 
lombaires de certains Edentes, que deux vertebres sont articulees entre elles par un double tenon entrant dans 
une double mortaise. La seule difference, c’est que les facettes de tenon et de la mortaise superieurs sont 
continues et forment entre elles un angle aigu.” The complex joints are, however, formed by different pro- 
cesses in the Reptilia and Edentata, as is explained in the text. 
