3yVO - HISTORICAL PALAZZAONTOLOGY. 
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 zmperfect 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 in zoological rank ; and some of the 
more striking examples of this may be here alluded to. 
Amongst the £chinoderms, for instance, the two orders gen- 
erally admitted to be the “lowest” in the zoological scale— 
namely, the Crzzozds and the Cystotds—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 Palzeozoic 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 Palaeozoic epoch, and they do not become abun- 
dant till we reach Mesozoic times. Amongst the AZollusca, 
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 Palzeozoic period, 
whilst the former abound in Mesozoic and Kainozoic forma- 
tions. Similarly, the Univalves 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 Palzeozoic 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 Mollusca—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 Palzozoic 
