18 GENESIS OF THE ARIETIDA. 
has not been ascertained in any one form, so far as we know. The microsi- 
phonula occurred, as might have been expected, earlier than the true goniatitic 
stages, or goniatitinula, in those species which had the nautiloidean stage with 
ventral saddle also prolonged into the second septum, as in the Asellati figured 
by Sandberger, and (ron. atratus figured by Branco. The goniatitinula became 
distinguishable when the first ventral lobe appeared. This was undivided, as 
in the lower Anarcestes and in the Magnosellaride among Goniatitine. This 
stage is prolonged through one or more septa in the higher Goniatitine, and 
also in the Lytoceratinzs and Ammonitins, and the whorl also at this time 
strikes one as similar to Anarcestes, or depressed semilunar in section, as stated 
above, and in these the goniatitinula is completed. 
The duration of the nepionic period can in a general way be described 
as coincident in extent with the duration of the smooth shell, which is always 
found at the centre of the umbilicus, however much the shell may be subse- 
quently ribbed and ornamented. This period would of course include many 
more transformations than the goniatitinula, especially among the higher and 
later occurring species of the Mesozoic. 
Haeckel designated all of the progressive stages which succeeded the true 
ovarian stages and included the nepionic and nealogic stages, and their 
structural relations, under the term Metamorphology.’ This term is, however, 
somewhat indefinite and artificial when limited in this way, since the ovarian 
stages are necessarily of very different duration in distinct groups, and cannot 
be considered as the natural limit of the embryologic period. We should, as 
above stated, be disposed to think that some such limit as here proposed would 
be nearer to the true one, namely, to consider the typembryos as the last of 
the true embryologic stages. This nomenclature would enable an author to 
give an approximate idea of the stage at which the metamorphologic stages 
began in any type. Thus, they would have begun in Nautiloids with the asi- 
phonula, and in the absence of this among Ammonoids with the ceecosiphonula. 
In the absence of this last, if it is absent among the lower Sepioidea, the meta- 
morphologic stages, according to the same rule, would begin with the first stage 
immediately succeeding the protoconchial stage. Whenever this last is absent, 
as it certainly is among the highest of the Sepioidea having meroblastic ova, 
then its equivalent stage, which represents what is left of the veliger, should 
be taken as the last of the embryologic stages. 
As has been noted above, the nzpionic period is always smooth, and is 
visible at the centre of the umbilicus in most discoidal shells, and the demar- 
cation is therefore visible between this and the nealogic period; but, as can be 
observed on most specimens, an attempt to separate the characters of the latter 
from the characters of adults is attended by greater difficulties. It is, however, 
essential to distinguish the category of ephebolic or adult characters from the 
nealogic, because in each form of any series there are usually found certain novel 
characters, which appear for the first time in that particular series. These make 
their first appearance almost invariably during the ephebolic period. 
1 Morph. d. Organismen, II. p. 22. 
