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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 Jura and 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 PI. xi-xiii 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 PI. iv and ending with PI. 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 PI. iv-vi. 



