LIASSIC FORMATIONS. Ill 



teeth come nearer in character to those of Ich. 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 Chiroliyostinus in plate 3 of Hawkins's ' Great Sea 

 Dragons,' fol., 1842, agrees with Ich. breviceps, and the name might be adopted 

 were it not applied by that author as a synonym of Ich. platyodon. 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 Bucldandi. 



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, in 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. 



