370 HISTORICAL PALAEONTOLOGY. 



in accordance with this, the Cephalopoda constitute a higher 

 group than the Bivalve Shell-fish. As in the case of superiority 

 of structural type, so in this case also, it is not in the least that 

 the Oyster is an imperfect animal. On the contrary, it is just 

 as perfectly adapted by its organisation to fill its own sphere 

 and to meet the exigencies of its own existence as is the 

 Cuttle-fish ; but the latter lives a life which is, physiologically, 

 higher than the former, and its organisation is correspondingly 

 increased in complexity. 



This being understood, it may be repeated that, in the 

 main, the succession of life upon the globe in point of time 

 has corresponded with the relative order of succession of the 

 great groups of animals m zoological rank ; and some of the 

 more striking examples of this may be here alluded to. 

 Amongst the Echi?iode7y?is, for instance, the two orders gen- 

 erally admitted to be the "lowest" in the zoological scale — 

 namely, the Crinoids and the Cystoids — are likewise the oldest, 

 both appearing in the Cambrian, the former slowly dying out 

 as we approach the Recent period, and the latter disappearing 

 wholly before the close of the Palaeozoic period. Amongst the 

 Crustaceans^ the ancient groups of the Trilobites, Ostracodes, 

 Phyllopods, Eurypterids, and Limuloids, some of which exist 

 at the present day, are all " low " types ; whereas the highly- 

 organised Decapods do not make their appearance till near the 

 close of the Palasozoic epoch, and they do not become abun- 

 dant till we reach Mesozoic times. Amongst the Mollusca, 

 those Bivalves which possess breathing-tubes (the " siphonate 

 Bivalves) are generally admitted to be higher than those which 

 are destitute of these organs (the "asiphonate" Bivalves); and 

 the latter are especially characteristic of the Palaeozoic period, 

 whilst the former abound in Mesozoic and Kainozoic forma- 

 tions. Similarly, the L^nivalves with breathing-tubes and a 

 corresponding notch in the mouth of the shell ("siphonosto- 

 matous " Univalves) are regarded as higher in the scale than 

 the round-mouthed vegetable-eating Sea-snails, in which no 

 respiratory siphons exist (''holostomatous" Univalves); but 

 the latter abound in the Palseozoic rocks — whereas the former 

 do not make their appearance till the Jurassic period, and 

 their higher groups do not seem to have existed till the close 

 of the Cretaceous. The Cephalopods, again — the highest of all 

 the groups of ]\Iollusca — are represented in the Palaeozoic 

 rocks exclusively by Tetrabranchiate forms, which constitute 

 the lowest of the two orders of this class ; whereas the more 

 highly specialised Dibranchiates do not make their appearance 

 till the commencement of the Mesozoic. The Palaeozoic 



