86 BRITISH FOSSIL REPTILES. 



may be deduced as a necessary consequence, such deductions being determinative of the 

 relations of the whole ; so far as to give the Knower a power of predicating results, both 

 zoologically, as respects the affinity of the otherwise unknown whole, of which only a 

 part affects the senses, and physiologically, as respects the living powers of the whole and 

 the part such extinct organism played in the sphere of its existence. 



This necessary connection and interdependency of the links of structures consti- 

 tute the essential condition and attractive character of palaeontological science. The 

 subjects, nevertheless, of the present Chapter, constrain me to submit the question, 

 how far this science of ours has advanced towards sustaining the foregoing definition ; in 

 other words, to how much of organic Nature at large, or of particular organisms, it is so 

 applicable ? 



Let us suppose, for example, that no other part of the petrified frame of an Ichthyo- 

 saur had come to our hands than had reached those of Scheuchzer or of Bronn, — a few 

 vertebral centrums, for example, from the hind part of the trunk. Could we have otherwise 

 concluded than they did ? Certainly not, had our knowledge of the vertebral structures 

 been restricted to the same parts of the extinct and fossilised animal. 



Biconcavity of centrums is a pre-eminently piscine character, but not without excep- 

 tions in the class of Fishes, even in that great proportion of the species whose osseous 

 development has advanced to individualised bony segments of the spinal column. Such 

 an exception, e. g. we have in the opisthocoelian vertebrae of the Bony Gar {Lepidosteus). 



But no known kind of Fish possesses vertebral centrums with both upper and lower 

 transverse processes (' diapophyses ' and 'parapophyses '). The presence of these in 

 certain of the biconcave vertebrae of Icihyosaurus, bespeaks that of ribs having a two-fold 

 articulation with their vertebrae ; and such structure of rib implies a body-cage adapted 

 to the movements of expansion and contraction of its cavity, which cavity we infer, there- 

 fore, to have contained bags receiving air, and to have had associated movements for the 

 purposes of respiration. 



But this function raises the exerciser above the grade of the Fish, and the next ques- 

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

 warm-blooded ? 



The biconcavity of the vertebrae 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 vertebrae ? 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 conseqviently 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 



