INTRODrCTION. 
XV 
nautilus to the higher forms of the Goniatitince, due to its divided 
ventral lobe ; of Centroceras to Agoniatites, and of Suhclgmenia to 
Agoniatites, and also the better known example of the ChjmenincB 
of the Devonian and the Aturia group of the Tertiary”; but, 
as further remarked by Hyatt, these parallel series occur in such 
zoological and geological relations as to render any sequence or 
descent of one from the other improbable. He supposes rather 
that they originated independently of the direct influence of 
inheritance ” h 
It might perhaps have been supposed that a genus so rich in 
species as Nautilus is would have supplied many subdivisions or 
groups of species ; but after carefully considering the matter, I have 
come to the conclusion that such groups are in the present case 
unnecessary, first, because the relationship in which the species 
stand to each other is fuUy set forth in the remarks appended to 
the descriptions of the species, and, secondly, because such groups 
are apt to become very artificial, owing to the necessity for frequent 
change in the selection of the characters upon which they rest. It 
is true that there are often met with in the genus Nautilus assem- 
blages of species having many characters in common, which are, 
however, too variable to found genera upon. Thus, in the Jurassic, 
a group of species might be constituted having Nautilus striatus, J. 
Sow., as its typical member, and including N. intenneiUus, J. 
Sowerby, N. simillimus, Foord and G. C. Crick, X Young 
and Bird, and perhaps N. ornatus, Foord and G. C. Crick C The 
characters uniting these species are, general conformity in external 
shape and close agreement in ornamentation. 
In attempting to form groups of smooth-shelled species, we must 
shift our ground from ornamentation to similarity of shape. Thus, 
Nautilus lineatus, J. Sowerby, might be united with N. pseudolineatus^ 
Foord and G. C. Crick, N. pohjgonalis, J. de C. Sowerby, and 
N. glaher, Foord and G. C. Crick, to form another group. The 
relationship of these species is pointed out in the description of 
N. lineatus (see infrc\ p. 213). They are all characterized by 
^ “ Genesis of the Arietidae ” (Smithsonian Contributions to Knowledge), 1889, 
pp. 25-26. See also remarks on the genus Aturia at p, 341 of this volume. 
2 This group is equal at least in part to Hyatt’s genus Cenoceras\ type 
N. intermedins, J. Sow. (See Proceed. Boston Soc. Nat. Hist. vol. xxii. 1883 
p. 300.) 
