68 BRITISH FOSSIL REPTILES. 



The sclerotic plates, thirteen in number, have been compressed from within at the side 

 opposite to tliat exposed, and the parts wliich were abruptly bent upon the midpart of the 

 eyeball have been pushed into line, and fractured at the bend with the fore parts of these 

 plates. 



The mandible shows a specific variety in the proportions of its constituent elements. 

 The angular (Tab. XXV, fig. 1, 31), which in Ich. plaf^odon has less depth, and in IcJi. 

 communis much less depth than the surangular, opposite the back part of the orbit, has in 

 Ich. breviceps greater depth there, and it extends further forward, viz. within nearly one 

 fourth of the fore end of the ramus, instead of terminating within one third {Ich. 

 platyodon), or before it reaches half way to that end {Ich. communis). 



The maxillary is excluded from the external nostril by the junction of the pre- 

 maxillary (22) with the lacrymal (73). The malar (26) extends further forward in a 

 slender pointed form, in advance of the orbit, between the lacrymal and maxillary (21). 

 The hinder expanded sutural border of the nasal (15) is sculptured by some strongly 

 marked ridges and grooves. 



The swollen base of the tooth is impressed by longitudinal grooves, fewer in the 

 upper (ib., fig. 3) than in the lower ones (fig. 4); the enamelled crown shows finer longi- 

 tudinal lineations; and the fore border is slightly trenchant. The crown is relatively longer 

 and more slender than in Ich. trigonodon, and less compressed than in Ich. platyodon. The 

 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 Chiroligostinus 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 Bucklandi. 



b. Ichthyosaurus communis, Conybeare. Tab. XX, figs. 2, 5, 5'; Tab. XXIV, fig. 1; 



Tab. XXVI, 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 on that ground, 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 



