Values of the Stages of Growth and Decline. 877 
resting upon the inner side of the scar. It is proposed to call this 
stage the Czecosiphonula, since it is undoubtedly the primitive stage 
of that organ. The cecosiphonula may indicate the former exist- 
ence of an ancestral form having a central axis composed of similar 
closed funnels or cecal pouches.! 
The third Silphologic stage in Nautiloids was completed by a 
septum (the second in the apical part of the shell) having an open 
funnel extending apically and joined to a loose textured siphonal 
wall which reached down into and lined the cecum, thus forming 
a secondary closed tube. In accordance with the structure this has 
been named the Macrosiphonula. 
The protoconch was present in Ammonoids and also in Belem- 
noids, but in both of these orders it was calcareous. The tendency 
to form a calcareous shell, which first appeared in the apex of the 
conch of the asiphonula in Nautiloids, became by concentration of 
development inherited earlier in the Ammonoids and Belemnoids 
in the veliger stage, thus transforming what would otherwise have 
been a horny shell into a calcareous one. The protoconch was, 
however, not otherwise changed in external aspect and retained the 
usual egg-like shape of the univalve veligers of the Cephalophora. 
As in the protoconchs of other similar veligers of Gasteropoda, 
ete., and as a result of calcification, the protoconch became fused 
with the apex of the conch more intimately than in Nautiloids. In 
other words the asiphonula, ‘after transmitting a portion of its 
characteristics to the typembryos of the Ammonoids and Belem- 
noids, disappeared, having been replaced by the Caxcosiphonula. 
The septum of the cxcosiphonula was consequently also inherited 
earlier, and became a functional substitute of the apical plate serv- 
ing to close the aperture of the protoconch, and its cecum extended 
into the upper part of the otherwise empty protoconch, in place of 
occupying the first air chamber as in Nautilvids. This is a remark- 
able example of the law of concentration, but by no means excep- 
tional. The fourth larval stage of the Nautiloids was completed 
by the building of the third septum. This septum had a long 
funnel and attached porous wall, but the wall formed a true siphonal 
tube opening apically into the next section, the macrosiphon. This 
* See also similar remarks by Whitfield, Bull. Amer. Mus. New York, 
No.1, and Em ryol. Ceph. by the author, Bull. Mus. Comp. Zool., V. 
8, No. 5, p. 100 
