878 Values of the Stages of Growth and Decline. 
was the beginning of the small siphon and can be appropriately 
termed the Microsiphonula. The microsiphonula was the typical 
stage of nearly all the known genera of Nautiloids, beginning with 
the Orthoceratites of the Cambrian and found at the present time 
in Nautilus, and also in all Ammonoids and Belemnoids without 
exception. 
Fortunately the genesis of both macrosiphonula and microsi- 
phonula can be traced in the adult forms and silphologic stages of 
well-known fossils. The Crytocerina had a siphon which was 
macrosiphonulate probably even in the adult stage, since it increases 
in diameter throughout life.  Piloceras had a huge siphon hardly 
at all contracted in the adults of some species, but considerably 
lessened in diameter during the same stage in others. Endoceras 
had also a large siphon always more or less contracted in the 
silphologic or later stages. The uncontracted macrosiphonula 
occupied in this genus a number of air chambers varying according 
to the species, from a few to six or more. This was evidently due 
to the earlier inheritance or concentration of the tendency to 
decrease the diameter of the siphon first manifested in the adults 
of Piloceras. Sannionites was a genus in which the siphon was 
smaller than in Endoceras, and probably, though this is not yet 
ascertained, inherited the tendency to microsiphonulation at the 
` first septum at an earlier age than in Endoceras. None of these 
forms, however, attained a true microsiphon, since even Sanni- 
onites had the siphon filled by endocones and in the centre an 
endosiphon.. These organs entirely disappeared in true microsi- 
phonulate forms and, in fact, could have existed only within a large 
macrosiphon. j 
Nevertheless this tendency to decrease the size of the siphon 
resulted in the formation of a definite constriction. This constric- 
tion was inherited at earlier and earlier stages after its origin m 
the siphon of Piloceras, until it became constant perhaps in Sanni- 
onites and certainly in the Orthoceratide. The constriction marked 
the line between the larger and smaller siphon in the macrosipho- 
nulate forms, and, in becoming constant through concentration, it 
became invariably fixed behind the first septum between n 
cæcosiphonula and the smaller siphon. This smaller siphon, 
though still a macrosiphon in structure, as explained above, even use 
