TRANSACTIONS OF THE SECTIONS. 79 
the occipital condyle ; laterally their inferior edges slightly overlap the first wedge- 
bone; posteriorly they thin away, exposing the postero-lateral edges of the centrum. 
That part of the centrum which forms the middle of the upper half of the atlantal 
cup is hexagonal, and it has a small pit in its centre; posteriorly its articular face is 
three times as great (nearly as large as the articular surface of the body of the axis), 
and has a circular depression in the middle. The wedge-bone forms the lower half 
of the articular cup, and has been produced behind into two long processes, the bases 
of which only remain, 
The neural spine of the axis is long, and much thicker than that of any of the 
succeeding cervical vertebree ; the apex is broken off in this specimen. The neur- 
apophyses are separated from the centrum by a distinct suture; and an oblique ridge 
connects on each side the anterior with the posterior zygapophysis. The antero-inferior 
edge of the axis is bevelled-off, forming an articular surface for the second wedge- 
bone; and the basal portions of two cervical ribs remain attached to the anterior 
lower part of the centrum: they must have partly articulated with the second wedge- 
bone. 
The axis of Plesiosaurus Etheridgii, lately examined by Prof. Huxley, agrees en- 
tirely with that of this species; but there are some important modifications in the 
structure of the atlas. 
Prof. Huxley describes the atlantal cup in this species as being divided by a tri- 
radiate mark into three portions—one inferior and two lateral and superior. The 
inferior piece corresponds with the lower half of the atlantal cup, or the anterior sub- 
vertebral wedge-bone; and the two supero-lateral pieces, with the neurapophyses in 
the specimen first described; but their bases are much more developed. 
There is a small circular bone in the centre, which Prof. Huxley considers to belong 
to the os odontoideum; it is the anterior articular face of that bone, and corresponds 
to the hexagonal bone in the middle of the upper half of the atlantal cup in the 
former species,—the difference in position being caused in this species by the great 
development of the supero-lateral pieces or bases of the neurapophyses. 
The postero-lateral edges of this bone are greatly developed, forming a rounded 
ridge on both sides of the posterior part of the atlas: the extraordinary development 
of this part of the bone is the most remarkable feature in the atlas and axis of this 
species. 
: We now come to the species first described by Prof. Owen. We have two speci- 
mens, in the Woodwardian Museum, of this species, both from the Kimmeridge Clay 
of Haddenham, near Ely; the larger of the two was figured by Prof. Owen in the 
* Annals of Natural History,’ vol. xx. 
The neural arches in both specimens are broken away; and the bodies of the two 
vertebrae have so coalesced, that the original line of separation is scarcely visible. 
The neurapophyses and cervical ribs of the axis have become anchylosed to the bodies 
of that vertebra. The posterior half of the bottom of the neural canal in the atlas is 
formed by the true centrum of that vertebra; but in the anterior half the bases of the 
neurapophyses have spread over the centrum, and have united at the medial line, 
On the upper part of the atlantal cup, a groove indicates the position of the original 
suture between the bases of the neurapophyses and the lower part of the atlantal cup. 
In the larger specimen there is a trace of the suture which separated the anterior 
subvertebral wedge-bone from the upper part of the atlas, but which is absent in the 
smaller specimen. 
In the atlas of the three species of Plesiosawrus we have now considered, the 
anterior articular face of the atlas is made up of four bones: of these, the os odon- 
toideum is the most variable in size. Prof. Owen correctly assigned to it a large share 
in the formation of the atlantal cup in the Kimmeridge Clay species. It forms about 
a third of the upper half of the cup in the young unanchylosed specimen, and in P, 
Etheridgit is extremely small. Its position varies: in the young specimen it forms 
the base of the neural canal of the atlas; in the Kimmeridge Clay species it is over- 
lapped by the expanded bases of the neurapophyses ; and in P. Etheridgii it occupies 
the centre of the cup. The bases of the neurapophyses of the atlas are most deve- 
loped in this species, and least in the Kimmeridge Clay species; in all cases the an- 
terior subvertebral wedge-bone forms a large portion of the atlantal cup. That the 
suture between this bone and the os odontoideum, in the atlas of the species figured 
