411 
apex, and at least as long ventro-dorsally as the same. In other 
words, the aperture of the protoconch in Nautiloidea was narrow 
and elongated vertically, while that of the Ammonoidea in all hay- 
ing cylindrical, straight or loosely coiled young shells, was an open 
tube, as happens in Clarke’s Orthoceran form, in Bactrites and in a 
number of Goniatitinze as shown in the figures. 
In most groups of Goniatitinze and the other suborders of Am- 
monoidea which, asa rule, have invariably closely-coiled first whorls, 
the effect of contact is to produce immediately a deep, contact fur- 
row and an almost entire obliteration of the umbilical perforation 
between the neck of the protoconch and the nepionic volution. 
Two funnel-shaped openings are left on either side, as shown in fig- 
ures on Pl. ii, and these represent the more complete perforation 
present in all Nautiloidea and in the earliest forms of Goniatitinz 
among Ammonoidea. The probable position of the aperture of 
the protoconch has been indicated in Lmdbryology of Fossil Cephalo- 
pods, p. 110, and in Pl. iv, Fig. 1, and this information, gathered 
from sections, agrees well with the figure given by Dr. Brown of the 
supposed aperture of Baculites which is reproduced in outline, Fig. 
cy rol ad ea he 
The growth of this form out of the protoconch, as in Bactrites, 
must have beén quite different from that of the true Nautiloidea. 
Nevertheless it is obvious that as the animal grew outside of the 
limits of the protoconchial aperture, it began to build the shell of 
the apex of the conch and the first living chamber. This was the 
ananepionic substage and it in part more or less resembled in some 
of its essential characteristics and for a short time, the aseptate, 
apical living chamber of the Nautiloid, but this resemblance must 
have been transient and much accelerated. 
After or during the building of this external skeletal tube it 
became practicable for the animal to lift itself, or, more properly 
‘speaking, to progress by growth out of the protoconch, and the 
next step can be seen in Branco’s Fig. 10, Pl. iii, and the details in 
my Fig. 7, Pl. iii, both of which, and others also given, show that 
the bottom of the caecum occupied the aperture of the protoconch 
and is formed, as in Nautiloids, of the closed funnel of the first 
septum. It is therefore inherited earlier, according to the law of 
tachygenesis, since the first septum and the czecum occupy the same 
position with relation to the protoconch as the scar or cicatrix in 
* Proc. Acad. Sci. Phil., 1892, Pl. ix, Figs. 5 and 10, 11. 
