590 
the more loosely coiled, and the more specialized show that they 
were derived from forms which were tightly coiled. In other words, 
the tendency to closer and closer coiling gains in the organization 
of the different genetic series, and is manifested more intensely in 
the young of more specialized forms and makes them coil more 
quickly and closer. In general, it is also easily seen that after the 
trunk forms die out in the Trias, as explained in the Introduction, 
page 370, and it is not possible for any new genetic series to be given 
off from these, this tendency has greater force. In the Juraand Cre- 
taceous the shells are exclusively nautilian, and even the nautilian 
shells with very large perforations, common even in the Triassic, 
have entirely disappeared. 
In the shells shown on Pl. xi-xii there is not one that has a 
really large umbilical perforation and a free cyrtoceran apex, such 
as is seen in so many of the Silurian, Devonian, Carboniferous, 
and even in some Triassic shells. All of these transitional forms 
disappear with the trunk forms, and the same fact is true of the 
Ammonoidea. ‘The transitional forms disappear in the Devonian at 
the same time with Bactrites, the radical straight form of this 
order. With regard to special series, it becomes more difficult to 
show agreement between chronology and _ bioplastology on account 
of the deficiencies in the records of collected forms and the gen- 
eral tendency of radical species to persist and be found either on 
exactly the same level with their descendants or even to outlive 
them and be present in later faunas. 
In studying the coiling of nautilian shells one is struck by the 
fact that the ana- and the metanepionic substages are comparatively 
straight. They are not really straight, as has been explained above, 
but their comparatively straight aspect, in contrast with the suc- 
ceeding stages of development, is noticeable. 
At the end of the metanepionic substage the curvature is apt to 
be suddenly altered, bending more rapidly inward. This is what I 
have called the gyroceran bend, because it is the first indication 
that the shell is a true nautilian form. If one compares the length 
of the ana- and metanepionic substages in the different plates begin- 
ning with Pl. iv and ending with Pl. xiii, it will be seen that there 
is a notable decrease in the comparative length of these two sub- 
stages when the umbilical perforations become very small, and the 
same is true of the species of the Calciferous and Silurian, which 
have small umbilical perforations, as shown on Pl. iv—vi. 
