LIASSIC FORMATIONS. 
Ill 
teeth come nearer in character to those of IcIl. communis^ but are relatively larger, and 
fewer in a given extent of the jaws, than in that species. 
In all the characters above defined as differentiating the present species from those with 
which it is compared, and to which it makes the nearest approach in the Ichthyosaurian 
series, the skull figured as that of Chiroligodinus in plate 3 of Hawkins’s ‘ Great Sea 
Dragons/ fob, 1842, agrees with Ich,. breviceps, and the name might be adopted 
were it not applied by that author as a synonym of Ich. plafyodon. Besides the 
Dorsetshire locality above named, Ichthyosaurus breviceps has been discovered in the 
Lower Lias in the neighbourhood of Brownish, Glastonbury, Somersetshire, in the Zone 
of Arietites Bucklandi. 
b. Ichthyosaurus communis, Conybeare. PI. XXIV, figs. 2, 5, 5'; PI. XXVIII, fig. 1 ; 
PI. XXX, figs. 3, 4, 5. 
The name was suggested by the evidences of this species being the most numerous 
that, at first, came to hand ; but subsequent acquisitions seem to show another species to 
have a better claim, at least in the locality of the Lias formations in the South-west of 
England. 
In Ichthyosaurus communis the length of the skeleton is about five and a half times 
that of the skull, and the length of the ‘ snout,’ or upper jaw, anterior to the orbit, is 
three and a quarter times that of the orbit 
Of great breadth posteriorly, the skull narrows to' the fore part of the orbits, thence 
the upper jaw contracts rapidly, afterwards gradually, to the anterior almost pointed 
end. As it advances the upper jaw becomes subcompressed. In profile, after the 
concavity due to the sinking of the cranium anterior to the orbit, the line goes straight 
to near the end of the upper jaw, where it rapidly sinks to the alveolar border. 
The chief characters of the present species are afforded by the teeth and the pectoral 
paddles. 
The teeth (PI. XXIV, figs. 5, 5') are more numerous and smaller than in Ich. brevi- 
ceps, but, ill comparison with the majority of the known species, are proportionately large. 
They have an expanded or ventricose root, contracting to a conical, slightly aduncate 
crown, with a subcircular transverse section. The apex is subacute, but there is no 
coronal trenchant margin ; the enamel is impressed by fine longitudinal grooves, with 
intervening ridges. These finer ridges are somewhat abruptly divided from the coarser 
ones of the root by a smooth tract marking the base of the enamelled crown. Viewed in 
the series the teeth seem to taper less regularly, often more quickly, to the apex than in 
other species. The upper jaw bears on each side from forty to fifty teeth, of which 
sixteen or eighteen may be implanted in the maxillary bone, the rest in the premaxillary. 
Each ramus of the mandible may support a few teeth more than those opposed to them 
in the upper jaw. 
