14 GENESIS OF THE ARIETIDZ. 
more primitive forms of several silurian and devonian species of Goniatitine, and 
this is notably the case in Bactrites which has a straight shell. In these primitive 
forms the apertures of the protoconchs must have been less contracted than in 
most Nautiloids, 
The apex of the conch did not expand so fast as in Nautiloids, but was more 
nearly of the same diameter as the neck of the protoconch, and often remained 
tubular for a considerable portion of the nepionic period. This was especially 
evident in the more open whorls of the anarcestian larve, figured by Sandberger, 
Barrande, and Branco. Among the close-coiled forms of paleozoic species, and in 
still later occurring genera, the protoconch itself became depressed, and a deep 
dorsal constriction resulted from the abruptness with which the apical part of the 
conch turned in upon the inner (dorsal) side of the protoconch. 
The calcareous nature of the shell, the depressed form and transversely con- 
stricted aperture, and the closer union of protoconch and conch among Ammo- 
noids, separated the young apparently so widely from those of Nautiloids, as to 
lead Barrande, Munier-Chalmas, and Branco to deny that transitions occurred 
between them. Another distinction of importance was, that the aperture of the 
protoconch was closed, not by an apical plate, but by the first septum. In other 
words, the asiphonula of Nautiloids disappeared as a distinct nepionic stage, 
and the cxcosiphonula took its place in the development of the young among 
Ammonoids. ‘This fact led Branco in his masterly work on the early stages to 
assert, in common with Barrande and Munier-Chalmas, that the protoconch of the 
Ammonoids was the homologue of the apex of the conch and first air-chamber in 
the Nautiloids. Certainly the calcareous shell and the position of the first sep- 
tum and cecum appear to be in favor of their view. 
On the other hand, the student of embryology will be slow to admit that the 
resemblances of the protoconch in Ammonoids to the veliger shell has no mean- 
ing. Ifit have any meaning at all, and can be compared with the protoconchs 
of the Cephalophora during the veliger stage, then during the whole of that 
stage the typembryos of Ammonoids, like all other veligers, could not have had 
a siphonal cecum or siphon. This is insured by the emptiness of the protoconch, 
the siphonal cecum being present only in the aperture, and not penetrating far 
back into or resting upon the first formed plate of the protoconch, as in the first 
air-chamber of Nautiloids, 
Another argument in favor of the view here advocated is the general fact, 
cited in the paper quoted above, upon the “ Values in Classification of the Stages 
of Growth and Decline,” that the typembryos, to which class of forms the veli- 
ger belongs, cannot be said to have the essential characters of any specialized 
division, like the Cephalopoda, but have to be compared with remote and gen- 
eralized types from whom their principal characteristics were inherited. 
The authors quoted above, holding the view that the protoconch was the 
homologue of the first chamber and apex of Nautiloids, necessarily rejected our 
theoretical explanation of the presence of the first septum and cecum in the 
aperture as due to acceleration of development. 
Nevertheless, this explanation still seems to us correct, and we have now a new 
