74 GENESIS OF THE ARIETIDA. 
shape, —a fact accordant with the more specialized structure and more acceler- 
ated development of the species in this genus. 
In Arnioceras' the goniatitic helmet shape was replaced by a purely psiloceran 
helmet shape on the third whorl ; and this was retained throughout life in Arn. 
miserabile,’ but lasted for a more limited period in Arn. semicostatum, and was then 
followed by a flatter and broader abdomen, the sides becoming slightly divergent, 
as in Arn. obtusiforme ;* and this condition was often maintained throughout the 
adult stage. 
The broad abdomen and divergent-sided whorl, which came out in only a few 
species of Vermiceras and Arnioceras, and was not very strongly marked in them, 
became in Coroniceras characteristic of the young at an early stage It is a 
significant fact favoring our theory, that in the arnioceran-like forms of Cor, 
kridion it did not replace the more compressed whorls of the arnioceran ancestor 
until a late stage of growth. In other species of Coroniceras, however, the 
broad abdomen and divergent-sided whorl replaced the laterally compressed, 
helmet-shaped whorl inherited from Arnioceras, as in Cor. Jatum.’ All the species 
of Coroniceras did not have this stage. It was in its turn more or less replaced, 
in some of them, by the acceleration of other characters, as will be shown 
farther on. 
The law of succession in anagenesis, therefore, is, that progressive species in each sepa- 
rate genetic series were the direct descendants of progressive varieties or forms. The facts 
consequently are in strict accord with the theory of descent with modification, and with the 
law of heredity, that lke tends to reproduce like. 
Coroniceras was not derived from Arn. semicostatum directly, but indirectly, 
through the more highly specialized forms of Arn. kridioides and Oor. kridion. It 
was not the varieties of Cor. kridion with arnioceran characteristics most com- 
pletely developed which led into Cor. rotiforme, but those with divergent-sided 
and highly specialized pile, keel, and channels. So in Cor. rotiforme with refer- 
ence to Cor. datum, and also in this last with reference to Oor. Bucklandi. These 
are the purely progressive forms; and their connection with ancestral species 
occurred through progressive varieties. 
CATAGENESIS,’ OR THE GENESIS OF RETROGRESSIVE CHARACTERS. 
Many large specimens of the species noted in the preceding remarks had 
narrow abdomens, and the sides converged outwardly. Thus, in what is often 
mistaken for the full grown adult stage of Caloceras an acute helmet shape 
appeared, as in some varieties of Cul. Johnston’, tortile, Liasicum, and nodotianum. 
This was certainly not, as usually stated by paleontologists, due to a retention of 
the psiloceran form. It took place after the intermediate or progressive stages 
in which the abdomen had become widened, more or less flattened, and the sides 
1 Embry. Ceph., pl. ii. fig. 8, 9. 2 Pl. ii. fig, 4-7. 
8 Compare above with semicostatum, pl. ii. fig. 10 and 15. © bl, iid 8: 
© Pl iy fig,-22 4 tp), ivedigy 13 ple vie fig, 6; eo Pl ae figs 20; 
7 Kara, downwards; Téveows, descent by birth. 
> ae 
