26 GENESIS OF THE ARIETIDA. 
equivalents, however close, have been derived or carried across the genetic lines 
of descent from an equivalent representative species of one branch to that of 
another. Nor could the similarities of such forms have been derived in any 
series from the radical species, because involved whorls, keels, channels, etc. did 
not exist in the discoidal stock forms. Parallel series and equivalent forms, also, 
occur often in such zodlogical and geological relations that any sequence or 
descent of one from the other is improbable; as, for example, Aturia of the 
Tertiary, and Clymenine of the Devonian; or Centroceras of the Devonian, 
or Subclymenia of the Carboniferous, and Agoniatites which began in the 
Silurian. 
These facts speak with great force for the continuity in descent of the dis- 
coidal shells, and for the existence of a primitive trunk line of generalized 
radicals, beginning with the earliest times and lasting into the Jura. The uni- 
versality of the phenomena leads at first to the supposition that we can account 
for morphological equivalence of species in different series by some invariable 
law of growth, such as is evidently the cause of the more exact parallelisms 
which occur between different individuals of the same species. We might con- 
sider each species as representing a hereditary grade of structure in the develop- 
ment of a series, just as any period in the life of an individual would represent a 
stage of development, inherited from some ancestral form. 
We were led into this error at first, but it is an inadmissible supposition in 
the light of the facts given above. These show, that the representative forms 
are absolutely new forms in their respective genera or groups, possessing char- 
acters not found in the stock or chronological trunk of discoidal radicals, and 
their resemblances are therefore homoplastic, and not homogenous. 
There are also many kinds of series among fossil Cephalopoda, and in some 
of these forms similar to those of the Ammonoids and Nautiloids are not pro- 
duced, as in the Sepioids and Belemnoids. In these orders entirely new modi- 
fications accompanied equally complete changes in habits and habitat. The 
crawling and shell-covered, littoral, radical Orthoceras has in these orders be- 
come changed into a swimming and predatory mollusk, the shell having become 
internal. It seems evident in these cases, that the forces of the surroundings 
and new habits deflected the Sepioids and Belemnoids from the more normal 
course taken by the Nautiloids and Ammonoids, and thus made the repetition of 
form or equivalence in the shells impossible, except very rarely, and then only in 
a very limited sense. Such, for example, are the similarities which exist between 
the internal shell of Spirula and the external shell of Lituites, or between the 
pseudo shell of the female Argonauta,’ and the true external shell of one of the 
compressed Ammonitine, like Cosmoceras or Hoplites. 
The disappearance of the siphon in the Sepioids, and the naked young of the 
existing forms of this order, show that too much weight can hardly be given to 
the modifying and eventually controlling influence of changes of habit, or, what 
is the same thing, the effects of the surroundings in any new habitat, whether 
1 See Evolution of Cephalopoda, Science, TIL, No. 52, 58, 1884; Foss. Ceph. of Mus. Comp. Zool., Bul- 
letin, I, No. 1; Proc. Am. Ass. Ady. Sci., XXIII., 1883, p 341. 
