862 THE JOURNAL OF GEOLOGY. 
in the Cephalopod shell, he says: ‘All their parts, the shell proper, 
the siphuncle, the septa and the sutures are in correlation with each 
other and together make an index to the life history of the individual, 
which is unequaled in some respects among other existing or extinct 
animals.”’ From the study of a single perfect specimen of a Cephalo- 
pod shell can be determined the particular characteristics of the indi- 
vidual at any stage of growth, from the embryonic condition represented 
by the protoconch, to the retrograde metamorphoses through which it 
may pass during old age. 
The two sub-classes of the class Cephalopoda, Tetrabranchiata and 
Dibranchiata, with the four orders Nautiloidea, Ammonoidea, Belem- 
noidea and Sepioidea, are shown to have been differentiated from 
some common primitive stock, each order being specialized for its own 
peculiar habitat. ‘The prominence of the depression or sinews in the 
aperture of the shell is correlative with the prominence of the hypo- 
nome or swimming organ of the Cephalopod, and therefore signifies the 
relative power of the animal as aswimmer. An open aperture is cor- 
relative with powerful arms for crawling on the bottom, while forms 
possessing shells with more or less constricted apertures must have 
had more or less limited powers as crawlers. In relation to the differ- 
entiation of the four orders the author says: ‘The effor¢s of the Ortho- 
ceratite to adapt itself fully to the requirements of a mixed habitat of 
swimming and crawling gave rise to the Nautiloidea ; the eforts of the 
same type to become completely a littoral crawler evolved the Ammon- 
idea. The successive forms of Belemnoidea arose in the same way. 
But here the ground-swimming habitat and complete fitness for that 
was the object. The Sepioidea, on the other hand, represent the higher 
aims as well as the highest attainments of the Cephalopods in their 
evolution into surface-swimming and rapacious forms. We cannot 
seriously imagine these changes to have resulted from intelligent effort; 
but we can with Lamark and Cope picture them as due to efforts on the 
part of the animal to take up new quarters in its environment and thus 
acquire habits and structures suitable to the changed physical require- 
ments of its surroundings and this position is better supported by facts 
than any other hypothesis.” 
The relationships of the Nautiloidea and Ammonoidea are next 
discussed. Many investigators have endeavored, and have failed, to 
arrange the palezozoic Nautiloidea in a single series, passing progres- 
sively from earlier straight forms, through arcuate to the closely coiled 
forms. Professor Hyatt is led to believe through his investigations, 
