Values of the Stages of Growth and Decline. 879 
Sannionites, was undoubtedly transitional to the true microsiphons 
of the Orthoceratide. 
The cæcosiphonula was in all Orthoceratites, which are other- 
wise similar to Endoceras, confined by concentration of development 
to the first air chamber, and a true microsiphonula appeared at an 
early stage as an open narrow tube. This was similar to the siphon 
of the vast majority of all succeeding forms of both Nautiloids and 
Ammonoids. According to the classification here advocated, the 
stages preceding the microsiphonula, viz.: the asiphonula, 
cecosiphonula and macrosiphonula, became silphologic stages in 
all the groups of Cephalopoda descending from the radical 
- Endoceratide. | Microsiphonulation became silphologic in the 
Orthoceratide, and the smooth shell which they evolved was 
subsequently inherited among Nautiloids, Ammonoids and Belem- 
noids during the younger stages in all the species of these orders. 
Other forms, with depressed and involved whorls, were introduced 
in the main stock of radicals among Goniatitinse, and were modi- 
fications of the smooth cylinder of the simpler Orthoceratide with 
its microsiphon. These in turn became the proximal radicals of 
derivative groups. Thus the Anarcestes'! among Goniatitine 
became the radicals of the Ammonoidea, and the smooth silphologie 
Stages of all Ammonoids after the expiration of the Devonian 
were like the adults of these lowest forms of Goniatitine. This 
later acquired silphologic stage has therefore been styled the 
Goniatitinula, 
It has also been found, that in tracing the descent of forms with- 
in smaller groups, sub-orders, families, and genera, it is practicable, 
as in the case of the family of Endoceratide, to prove that 
characteristics usually appear first in adult stages and are then 
inherited at earlier and earlier stages in successive species of the 
Same stock, whether they occur on the same horizon, or in different 
horizons. The adolescent or Nealogic? stages are of as great import- 
ance for tracing the genealogy of small groups as are the silpholo- 
gic characters in larger groups. Thus one can speak in definite 
terms of the relations of the nealogic stages, and their meaning 
and importance in tracing the genealogy of families and genera, 
* Gen. Ceph. Proc. Bost. Soc, Nat. Hist., V. 22, 1880, p. 305. 
? Neadyc, youthfulness. 
