NOMENCLATURE OF STAGES OF GROWTH AND DECLINE. 13 
functional siphon in these shells. Gerard Holm! first called attention to the 
interesting character of the young stages of the siphon in Endoceras, and has 
shown this organ to have been very large even in the young, having not only 
a cecal beginning, as in other forms, but in several species having a swollen 
or macrosiphonulate form which endured throughout several septa. In speci- 
mens now in the Museum of Comparative Zodlogy, at least six septa were built 
before any signs of contraction began to appear. In other cases figured by Holm 
the siphonal cecum, though very large as compared with that of Orthoceras, was 
attached to the first septum, as in all the shells so far known from that group, 
and occupied only the first air-chamber. We should suggest to those having 
materials for study, that the shells having this last character are very likely not 
true Endoceratites, but perhaps the young of species of the genus Sannionites, 
which, according to the classification followed by the author, is a genus distinct 
from Endoceras, because the species possess a much slenderer siphon. Whatever 
the fate of this suggestion, it is plain that transitional series exist in this group 
between Sannionites and Cyrtocerina or Piloceras, and that gradations occurred 
also in Piloceras, which show that contraction of the siphon began first in adults, 
and then, according to the law of acceleration, was inherited in the nealogic 
stages of immediate descendants, and finally became nepionic in the smaller 
siphoned species of the genera Endoceras and Sannionites. This tendency to 
contraction in the diameter of the siphon indicated the beginning of a series of 
transformations which accompanied a decrease in size of the fleshy siphon, and 
other correlative transformations, such as the decrease in length of the funnels, and 
the contraction and straightening out of the calcareous endocones, so as to form 
the walls of a tubular siphon. In other words, as the siphon contracted, the func- 
tional endosiphon formed by the open and extended tips of the endocones was 
finally brought into line with the funnels, and together with them formed the 
microsiphon, which is consequently a degraded modification derived from the 
funnels, endosiphon, and endocones of the Endoceratidee. The Orthoceratide and 
all the remaining forms, with some notable exceptions which we shall take up 
and describe in future papers, had a microsiphon. The whole microsiphon formed 
a continuous open tube of narrow diameter, reaching from the last septum to the 
nepionic septa, which represented the macrosiphonula. Doubtless the duration 
of nzepionic stages will be found to vary somewhat in ancient forms, but the 
indications, so far as known, are in favor of the theory that the vast majority of 
even ancient forms had a microsiphon, which was developed comparatively early 
in the life of the animal. 
The nealogic stages of succeeding groups would be very interesting if 
there were space to describe them, but we shall have to illustrate this part of 
our work among the Ammonitine. The protoconchs of Ammonoidea, including 
the genus Bactrites, had, as remarked above, globose forms with calcareous 
shells, and these shells were continuous with the apex of the conch, but the 
aspect of the junctions was quite distinct from those of Nautiloids. The con- 
striction between them and the apex was very slight in the uncoiled young of the 
' Dames et Kayser, Paleontol. Abhandl., [II., Part I. 
