756 
PROFESSOR OVFEN OxN THE MEGATHERIUM. 
Ichthyosaurus derives additional strength and fixation from hypapophyses autoge- 
nously developed, at the inferior interspace between the occiput and atlas, and at 
those between two or three succeeding cervical vertebree* * * § . The so-called ‘body of 
the atlas,’ in recent Saurians, Birds, Mammals and Man, is the homologue of the first 
of these subvertebral wedge-bones, and represents only the hypapophysial or lower 
cortical part of such body. The odontoid process of the second vertebra is the cen- 
tral and chief part of the body of the atlas-f-, but not the whole body, as Cuvier de- 
scribes in the fimbriated Tortoise;}:. 
The most interesting and instructive steps in the transition of this main part of 
the centrum of the atlas to the modified condition which it presents in Man, and 
which is signified by the term ‘ processus odontoides ’ in Human Anatomy, are exem- 
plified in the order Chelonia. In the Australian Long-necked Tortoise {Hydraspis 
/owg/co/Z/.y, Bell), I find the centrum (cc, figs. 57, 58, Plate LII.) completely an- 
chylosed with the neural arch {n) of the atlas, with the exception of the hypapophysis 
{hy), which is autogenous in this vertebra and preserves its distinctness. In the Soft- 
turtles ( Trionyx) [ih. figs. 52, 53 and 54) both the centrum and hypapophysis of the 
atlas maintain their primitive individuality; the centrum (c) is relatively shorter 
than in the Chelys and Hydraspis ; it presents a subcubical form ; has a small sub- 
circular surface (c, fig. 55) on its lower and fore-part for articulating with the hyp- 
apophysis ; above this it expands transversely and presents a convex articular surface 
adapted laterally to the bases of the neurapophyses, and, mesially, completing the cup 
for the occipital condyle, and filling up the vacancy shown at c, fig. 53 ; the back- 
part presents a concave surface (c, fig. 56) which articulates with the anterior con- 
vexity of the centrum of the dentata. The neurapophyses of the atlas (fig. 54, n) 
rest partly upon their proper centrum (c), partly upon its hypapophysis {hy)'. their 
bases nearly meet above the centrum, and their apices quite meet, uniting by suture 
above the neural canal (fig. 53). The degradation of the centrum of the atlas to the 
odontoid process of the axis is completed by its confluence with the centrum of the 
latter vertebra, and by the total transference of the neurapophyses of the atlas to the 
hypapophysis. Fishes, Batrachians and the extinct Enaliosaurians, manifest the typical 
state of the atlas, the Warm-blooded Vertebrates its aberrant state, the transitional 
modifications are traceable in the Chelonian, Lacertilian and Ophidian reptiles §. 
When the hypapophysis attains so complex a form as to be perforated and thus to 
constitute an arch protecting a vascular canal, it may readily be mistaken for some- 
thing more than the mere analogue of the ‘ hsemal arch’ of the typical vertebra; and, 
in my earlier investigations on that subject, 1 was led into the error || of regarding it 
* Sir Philip he Malpas Grey Egeeton, Bart., F.R.S., in Geol. Trans. 2nd Ser. vol. v. p. 187, pi. 14. 
t Annals of Nat. History, vol. xx. 1847, p. 92. 
1; Ossetnens FossUes, tom. v. pt. ii. p. 207^. 
§ See the description and figures of the atlas and axis of the Python in my ‘ History of British Fossil Reptiles,’ 
Part III. p. 136, pi. 3 {Ophidia), figs. 38 — 40. 
II Geol. Transactions, vol. v. 2nd Ser. p. 519, pi. 44^ figs. 2 and 3. 
