272 Messrs. Hancock & Atthey on Reptile- and Fish-Remains 
however, from the underside in both specimens, the entire 
form of this plate is developed. The arrangement and general 
appearance of the three plates are very similar to those of 
Loxomma Allmanni figured by Prof. Huxley in the paper 
already referred to; only the central plate is not so much pro- 
duced posteriorly and the lateral ones are much more elongated, 
agreeing apparently in this latter respect with Archegosaurus, 
in which Prof. Owen remarks that the lateral plates “ are 
shaped like beetles’ elytra’”*. Those of Pteroplax certainly 
resemble in form elongated elytra; but, unfortunately, these 
plates are not perfect: even in the best-preserved specimen 
only the posterior extremities are entire; they are rounded 
diagonally, the slope being apparently upwards and outwards. 
What remains of the most perfect plate measures upwards of 
seven inches in length, and three and a quarter wide. ‘That 
which seems to be the external margin is thicker than the 
inner or opposite margin; and here the surface-sculpture, 
which is like that of the other bones, is strongest. 
The central plate, which in Labyrinthodonts is usually 
rhomboidal, is peculiar in form: the two lateral angles are 
much produced, forming broad rounded lobes or wings 
(hence the generic appellation) ; their anterior margins have a 
sigmoidal curve extending to the anterior angle: the poste- 
rior margin is almost straight, but is a little produced in 
the centre, where there is a broad flat process extending 
backwards; this, however, is not perfect in either speci- 
men. Here the plate is thickest, and on the surface there 
is a strong sinuous ridge which extends transversely from 
side to side, just a little in advance of the posterior margin. 
When the lateral plates are in their proper position, their 
posterior extremities would, no doubt, rest against this ridge, 
the anterior extremities converging forwards. The central 
plate is upwards of four inches long and six and three-quarters 
wide. The surface is not sculptured in the usual manner, and 
must be almost entirely overlapped by the lateral plates. 
The other set of sternal plates is very imperfect ; the cha- 
racters, however, of the central plate are well developed; and 
it fortunately happens that while the right lobe of this plate, 
in the former specimen, is imperfect, the right lobe of this is 
quite entire, the other one being injured; so that, by the aid 
of the two specimens, the form can be perfectly restored. 
The two vertebree with the neural arches complete are in a 
very good state of preservation; the centrum is quite perfect 
In one, and almost perfect in the other; and between the two 
* Paleontology, p. 179. 
