ANAGENESIS. 13 
There is a similar succession in the evolution of the varietal and specific 
characteristics of the series. Thus, in Plate II., Fig. 10, 11, and 15 repre- 
sent the extreme varieties of Arn. semicostatum, the former with immature pile 
without channels, the second with well developed pile without channels, and 
the third with well developed pil and distinct channels.!| The young of Arn. 
tardecrescens, Fig. 19, shows that the derivation of this species was probably 
from the unchannelled forms of Arn. semicostatum similar to Fig. 11. The 
same is true of Arn. Bodleyi with reference to Hartmanni, as is shown by com. 
paring the young of the former, Plate II. Fig. 23, with the adult of Hartmanni, 
Fig. 17, which had very slight channels even in the adult, and this is still more 
apparent in the involute flattened form of Fig. 24, in which the channels 
were earlier developed. The close connection of all its characters, both of 
young and full grown, with its immediate ancestor, forbids us imagining an 
independent descent from a variety of any other species than MHartmanni, and 
the evidence is strong that it had no descendants: beyond its own species. 
The channelless variety of Arn. fulcaries, Fig. 26, gives similar evidence -of its 
genetic connection with the channelless varieties of Arn. Hartmanni, or if this is 
doubted, a more direct connection with Arn. semicostatum may be claimed ; but 
certainly there is no evidence for any connection with the channelled varieties 
of semicostatum or Hartmanni. 
The pilz began with psiloceran-like immature folds, after the keel appeared 
in the arnioceran varieties of Cor. kridion, and before the keel in the more acceler- 
ated development of other varieties of the same species. In some forms the 
pile were completed and became tuberculated before the channels appeared.’ 
This same confusion with regard to the time of the appearance of characteristics 
with relation to each other is a peculiarity of highly accelerated species, as 
before noted in Ver. ophioides. 
Such observations are of importance, since they enable us to understand that 
characteristics do not necessarily develop with invariable regularity. The usual 
order of their succession may be in a measure changed, or even reversed, when 
acceleration takes effect upon one character more than another. So far we have 
found this occurred only in species where all the principal characters of the series 
were undergoing exceptional acceleration. 
The common form of the younger stages of all the Ammonitins during the 
goniatitinula stage is shown in the plates.? The depressed goniatitic helmet shape 
was succeeded in Psiloceras by a laterally flattened helmet shape. In Caloceras 
the same form was succeeded for a very prolonged period, in species with flattened 
abdomens, like Cad. carusense,* by a stage in which the abdomen became broader 
and the sides slightly divergent. In Arn. ceras, as in Cal. carusense,> this often 
occurred at a much earlier period, replacing entirely the psiloceran helmet 
1 By accident these specimens were al] of different sizes. Thus they give false impressions. Although 
fig. 15 is older and larger than the others here figured, my observations were made on specimens of similar 
size and age. 
4 Plems fip., 9:20. 
8 Pl. i. fig. 4’a, for Psil. planorbe ; pl. vi. fig. 7, for Cor. Sauzeanum ; also in Embryology of Cephalopods. 
oP i ie Tey de, el shat 2OF 
10 
