LIASSIC FORMATIONS. 129 



tion of the Palaeontologist would be, whether the air-breather was cold-blooded or 

 warm-blooded. 



The biconcavity of the vertebras would sustain the first conclusion, and consequently 

 a reference of the extinct animal, so fragmentarily indicated, to the Reptilian, not the 

 Mammalian class. 



But what further insight into the nature of such Reptile could be gained by contem- 

 plation of a solitary centrum, or even of a series of vertebras ? With me no further step 

 could be taken toward a sure knowledge of the nature of the cold-blooded air-breather, so 

 partially indicated. A suspicion, at most, of an aquatic medium, and consequently of 

 limb-structures adapted to locomotion therein, might have crossed the mind. But a 

 complete reconstruction of the extinct animal, or certain knowledge of such, could only 

 be the result of acquisition and comparison of the anatomy of the cranium, as well as of 

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

 subjects of the foregoing pages of the present Monograph. 



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 Ichthyopteryyia 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 Pishes. 



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 hind 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 super- 

 aqueous atmosphere. 



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

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

 spicuously adapted to the detection of the finny prey as in our 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 squeezed humours within and with the contrac- 

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



