Head*
The head of the P. Dolichodeirus exhibits a
combination of the characters of the Ichthyosaurus,
the Crocodile, and the Lizard, but most
nearly approaches to the latter. It agrees with
the Ichthyosaurus in the smallness of its nostrils,
and also in their position near the anterior angle
of the eye; it resembles the Crocodile, in having
the teeth lodged in distinct alveoli; but differs
from both, in the form and shortness of its head,
many characters of which approach closely to
the Iguana.'j'
feet fossil skeleton, also in the British Museum, discovered by Mr.
Hawkins, in the lias at Street, near Glastonbury. At PI. 16 is also
copied Mr. Conybeare’s restoration of this animal, from dislocated
fragments, before any entire skeletons were found. The near
approach of this restoration to the character of the perfect skeletons,
affords a striking example of the sure grounds on which
comparative anatomy enables us to reconstruct the bodies of fossil
animals, from a careful combination of insulated parts. The
soundness of the reasoning of Cuvier, on the fossil quadrupeds
of Montmartre, was established by the subsequent discovery of
skeletons, such as he had conjecturally restored from insulated
bones. Mr. Conybeare’s restoration of the Plesiosaurus Dolichodeirus,
(PI. 16,) was not less fully confirmed by the specimens
above mentioned.
* See PL. 16, 17, 18.
f Mr. Conybeare, in the Geol. Trans, second series, vol. 1,
part 1, PI. 19, has published figures of the superior and lateral
view of a nearly perfect head of this animal. Our figure, PI. 18,
Fig. 2, represents the head of the specimen in the British Mu-
Neck.
The most anomalous of all the characters of
P. Dolichodeirus is the extraordinary extension
of the neck, to a length almost equalling that of
the body and tail together, and surpassing in the
number of its vertebrae (about thirty-three) that
of the most long-necked bird, the Swan : it thus
deviates in the greatest degree from the almost
seum, of which the entire figure, on a smaller scale, is given in
PI. 16. The head is in a supine position; the upper jaw is distorted,
and shows several of the separate alveoli that contained
the teeth, and also the posterior portion of the palate. The
under jaw is but little disturbed.
A figure of another lower jaw is given at PI. 18, Fig. 1, taken
from a specimen also in the British Museum, found by Mr. Hawkins,
at Street.
PI. 19, Fig. 3, represents the extremity of the dental bone of
another lower jaw, in the same collection, retaining several teeth
in the anterior sockets, and also exhibiting a series of new teeth,
rising within an interior range of small cavities. This arrangement
for the formation of new teeth, in cells within the bony
mass that contains the older teeth, from which they shoot irregularly
forwards through the substance of the bone, forms an
important point of resemblance whereby the Plesiosaurus assumes,
in the renovation of its teeth, the character of Lizards,
combined with the position of the perfect teeth in distinct alveoli,
after the manner of Crocodiles.
The number of teeth in the lower jaw was fifty-four, which, if
met by a corresponding series in the upper jaw, must have made
the total number to exceed one hundred. The anterior part of
the extremity of the jaw enlarges itself like the bowl of a spoon,
to allow space for the reception of the six first teeth on each side,
which are the largest of all.