The Early Stages of Bactrites. — Clarke. 41 
little unsymmetrical or directed to one side, very broadly 
sessile upon the end of the shell-tube, from which it is sepa- 
rated by a sharp constriction. Its form is uniformly globose 
or subspherical, without the slight diminution in diameter or 
tapering, which is seen in the protoconch of Orthoceras des- 
cribed from the same fauna, as well as in that of "Belemnites, 
as figured by Branco. In all these, however, t-he degree of 
constriction at the base of the protoconch is the same. Fig- 
ures 4 and 5. which represent the usual form of this body, are 
taken from specimens in which the original shell has been 
replaced; figure 6 is from a smaller individual and shows a 
decidedly broader constriction at the first septum ami a rela- 
tively less diameter in proportion to that of the shell-tube. 
This is a solid internal silicious cast, but the difference in re- 
tention will not explain the differences in size and form. 
Figures 4 and 6 have the same degree of enlargement, and 
while specimens like figure 6 are not uncommon, they all have 
a uniformity in size, at times the protoconch being even less 
clearly defined than here. These differences may be specific, 
or perhaps the smaller protoconchs and young shell-tubes may 
be parts of more fully grown shells, and have suffered dimi- 
nution in size from resorption. 
First septum. Frequentl}' the delicate protoconch is broken 
and such specimens have afforded means of determining the 
fact that the opening of the sipho in the first septum is dis- 
tinctly lateral, as shown in figure 7. This is an important 
distinction from the character of the first septum in Orthoce- 
ras, where the opening of the sipho is central. None of the 
specimens studied have afforded means of determining how 
the sipho begins, or whether ;i cicatrix exists upon the distal 
surface of the septum. 
Iiiifiii/ Shell-tube. The Oncoceras- or Gomphoceras-Miie 
swelling of the shell-tube directly above the protoconch is one 
of the most striking features of these shells. In specimens of 
B. gracilis it is highly developed and a persistent feature. 
The increase in diameter from the first septum upward is 
(piite rapid for a distance of two air chambers, and thence 
falls rapidly away. I speak of this as an Oncoveras-like ex- 
pansion, for its form, usually unsymmetrical, suggests that 
genus and may afford a key to it^ phyletic position. It may 
