LIASSIC ICHTHYOSAURS. 87 



the limbs and their sustaining arches ; and such has been the knowledge supplied by the 

 sulijects of the foregoing pages of the present Chapter. 



Here I may remark that instances of ancient extinct forms, manifesting a more gene- 

 ralised type, are more than ever worthy of note in the present phase of biological science ; 

 and the Iclithyopterygia contribute a welcome addition to this suggestive class of pheno- 

 mena. In the construction of their chief natatory organ for forward movement may be 

 discerned a combination of mammalian, saurian, and ichthyic conditions. In the great 

 length and gradual diminution of the caudal series of vertebrae may be noted the 

 saurian character ; the tegumentary expansion, unsupported by bony rays, recalls the main 

 feature of the cetaceous tail-fin, while its vertical position in the air-breathing Saurian 

 brings it in close parallel relation with the corresponding natatory propeller in the class 

 of Fishes. 



Nevertheless, the Ichthyosaur, as an aquatic air-breather, might be supposed to have 

 exchanged, at a loss, the disposition of its terminal fin in comparison with its aquatic 

 warm-blooded, fish-like successors ; but the pair of pelvic fins, wanting in all Cetaceans, 

 are superadded to the locomotive instruments in the Ichthyosaurs, and were, doubtless, 

 actively applied to bring the nostrils, when needed, within range of the superaqueous 

 atmosphere. 



But in almost every extinct natural group of animals peculiar conditions present 

 themselves. In no known cold-blooded Fishes was the visual organ so well, or so con- 

 spicuously adapted to the detection of the finny prey as in ovir present subjects. To 

 unusual size of eyeball, which in Dr. Buckland's experience sometimes reached that of a 

 man's head, was added a circle of concomitantly large bones — the ' sclerotic plates ' — of 

 form and structure in harmony with the requirements of the visual outlooks. Was a near 

 object to be detected, the retraction of the bony circle and contraction of its aperture, 

 surrounded by the laterally overlapping plates, would coincide with a concomitant con- 

 vexity of the cornea pressed upon by the sqiieezed humours within and with the contrac- 

 tion of the pupil — conditions concurring in the needed microscopic application of the eye. 

 If the distant expanse of waters was to be scanned, the resumption of the normal position 

 of the sclerotic ring and of the relaxed relations of the overlapping plates would coincide 

 with an expansion of the pupil and flattening of the cornea, whereby the eye would thus 

 acquire a telescopic range. Moreover, the wide transparent corneal aperture of the con- 

 spicuously large organ of vision would enable the predatory Saurian to take advatage of 

 the least amount of light penetrating the depths of its marine environment. 



