44 THE AMERICAN NATURALIST. [Vor. XXXV. 
but for convenience the adult or ephebic stage may be said to 
have begun when the compressed form, the greatest complexity 
of septa, and the rough sculpture of maturity are visible. This 
is the case at the distance of about seven centimeters from the 
larval coil, when the animal has by no means attained to 
maturesize. Further growth is then only increase in size and not 
progression of development. Old age, or the gerontic stage, 
shows itself in the obsolescence of sculpture and of the 
increase in size. Only a few specimens showing senile degen- 
eration were found, which is not surprising if we consider the 
small chance any of the lower animals have of becoming old. 
Phylogeny of Baculites. — This genus has usually been 
classed as an aberrant form under the Lytoceratide, but E. 
Haug ! says that the resemblance of the adult septa of Bacu- 
lites to those of Lytoceras is accidental; that Lytoceras in 
youth always has trienidian (three-pointed) septa, while Bacu- 
lites is always dicranidian (two-pointed). But the writer? has 
recently described the development of a somewhat degenerate 
species of Lytoceras, in which the septa are two-pointed in the 
earliest adolescent stage. This observation removes the objec- 
tion to the commonly accepted derivation of Baculites, and is 
especially interesting in view of the fact that Lytoceras in its 
old age often leaves the coila little way. The young stages 
of the species of Lytoceras studied by the writer are almost 
exactly like those of Baculites chicoensis, the latter showing 
only a greater contraction of the larval chamber, a premature 
ornamentation of the embryonic and larval shell, and a reduc- 
tion of the number of lobes and saddles. The only other 
studies on the development of this genus have been made by 
Dr. Amos P. Brown? on Baculites compressus. As compared 
with that species, Baculites chicoensis shows greater degenera- 
tion, for it leaves the spiral at the end of one revolution, while 
1 Les Ammonites du Permien et du 
vol. xxii, p. 410, 1894. 
2 The velopment of Lytoceras 
series, Geology, vol. i, No. 4, 1898. 
* On the Young of Baculites Com 
1891, pp. 159, 160; and The Develo 
Baculites Compressus Say, 
Trias, Bull. Soc. Géol. France, 3¢ sér., 
and Phylloceras, Proc. Cal. Acad. Sci., third 
pressus Say, Proc. Phil. Acad. Nat. Sci., 
pment of the Shell in the Coiled Stage of 
Proc. Phil. Acad. Nat. Sci., 1892, pp. 136-141. 
