Mr. W. King on the Tetrabranchiatc Ceplialopods. 277 



The development of so many genera of the fohate-plate cepha- 

 lopodous shells at a time when they were about to disappear, 

 would almost induce the notion that every form that could be as- 

 sumed had been tried to perpetuate them : but a grand organic 

 change was to arrest their evolutions : that change was to anni- 

 hilate them : — and thus the eve of the secondary epoch, which 

 had seen them luxuriating under every form, was destined to be a 

 witness to the final struggles of the Ammonidians ! 



Let us now turn our attention to the Nautilidians. It mil be 

 recollected that we left them revelling in the Carboniferous epoch 

 under the forms of Orthocerus, Cyrthocenis, Discus and Nau- 

 tilus. "With one exception, and at the close of this period, these 

 forms became suddenly extinct, and even the excepted genus — 

 the last, appears to have with difiiculty escaped the fate of its 

 congeners, since the deposits which were formed during the suc- 

 ceeding ages, the Permian and the Triassic, yield us but few spe- 

 cies. Afterwards the Nautilus seems to have completely reco- 

 vered from the check which it had previously sustained, as the 

 Jurassic and the Cretaceous rocks teem with new specific 

 forms. This state however did not continue, for the same devas- 

 tating influences which overtook the Ammonidians encompassed 

 the Nautiluses : the first were destroyed and the last siu'vived ; 

 but only to live in reduced numbers during the Tertiary epoch, 

 and to be reduced still more in our o^^^l. 



Like the Ammonidians of the Jm-assic and the Cretaceous pe- 

 riod, some of the contemporaneous Nautiluses appear under cha- 

 racters somewhat diiferent from those of their predecessors : for 

 example, the slightly sinuous edge which in general marks the 

 plates of this genus is lobed in such species as Nautilus sinuatus 

 and N. biangulatus ; the same character appears to have been 

 preserved in Nautilus Danicus, and it would seem to have arrived 

 at its maximum in the Tertiary Nautilus sipho and N. ziczac. 



We have now traced the Tetrabranchiatc Cephalopods through- 

 out their entire existence. We have observed them in one period 

 abounding to excess, and in the next to become considerably re- 

 duced ; then again to burst forth in countless numbers, and after- 

 wards to become all but extinct. 



Shall we conclude that the existence of but two species in the 

 present seas indicates an approximating termination to their ca- 

 reer ? A knowledge of their past history ought to make us pause 

 before we adopt this conclusion, for what have we to oppose their 

 re-entering another Jurassic period? — their again appearing in a 

 thousand forms ? — in short, what have we to disprove, that they 

 are still destined to sustain an important part in the future zoo- 

 logical revolutions of our globe ? 



