128 FOSSIL REPTILIA OF THE 



D. Conclusion. 



Although a study of the evidences of Ichthyopterygian organisation, of which, and its 

 modifications, as exemplified at the Liassic period, the results are given in the foregoing 

 pages, has left the impression, mainly, of the great additions which wait to reward subse- 

 quent cultivators of this field of comparative osteology, I am unwilling to quit it without 

 giving expression to some of the general notions which its cultivation has suggested. 



Palaeontology has been regarded, if not defined, as including a kind of knowledge of 

 parts, or of structures, in such degree interdependent that, any one being given, others 

 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 palseontological science. The 

 subject, nevertheless, of the present Monograph, constrains 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 opisthoccelian vertebra? 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 vertebras of Icthyosauras, bespeaks that of ribs having a two-fold 

 articulation with their vertebras ; 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- 



