512 ~ MOLLUSCA SUB-KINGDOM VI 
even in perfect shells, that good examples are rarely found, and when met 
with require careful preparation and close observation. 
In some Palaeozoic Nautiloids with large siphuncles (ndoceras, Actinoceras), 
the apical end of the siphuncle is solid and dilated to form the nepionic bulb 
(Hyatt), and this sometimes practically fills the camerae, and besides being 
very large in a number of succeeding chambers. The endosiphuncle expands 
near the apex in these genera, and forms a good-sized conical perforation or 
cicatrix, which is obviously open at its termination (Actinoceras, Nanno). 
Closely coiled shells have the apical part bent so as to enclose a vacant 
space (the wmbilical perforation) in the centre of. the whorls (Fig. 1070). 
This is present in all the Nautilowea having this mode of growth, although in 
some genera it is very minute. ‘The Nautiloid shell is invariably cone-shaped, 
but this may be straight or curved, or coiled in open or closed spirals, but in 
rare instances it is even screw-like, or similar to a Gastropod shell. Along 
with perfectly smooth shells, or those marked only with fine growth-lines, 
which in some rare cases may retain traces of their original coloration, there 
are others with external transverse ridges, keels, rows of tubercles or laminae ; 
but this ornamentation is of a simple kind, and never attains the degree of 
complexity observed among the more highly ornamented forms of Ammonoids. 
Classification.—Great importance has always been attributed to the external 
configuration and curvature of the shell in distinguishing genera, and the 
principal groups usually named Orthoceras, Cyrtoceras, Gyroceras, Nautilus, ete., 
have been founded upon such characters. Barrande emphasised in addition 
the shape of the aperture, direction of the funnels, and structure of the 
siphuncle, but considered these subordinate in most cases to the general 
form, and the majority of writers have followed his example. Hyatt, 
however, has regarded the general form and involution of the shell as 
relatively minor characters, and depends upon coincidence of structure, 
outlines of the aperture, and especially resemblances in developmental 
stages, as surer guides to the affinities of the species and characteristics of 
the genera. 
Terminology..—For sake of convenience, it is preferable always to speak of 
the embryonal shell as the protoconch, and the later or epembryonic stages of 
the shell as the conch, the term “shell” being really applicable to the entire 
external skeleton inclusive of the protoconch. The history of the individual 
and its shell can be divided into the following stages and sub-stages :—The 
embryo or protoconch ; the nepionic stage or infancy, represented by the apical 
part of the conch ; neanic stage or adolescent part of the more mature cone ; 
ephebic or adult stage of the same; and gerontic or senile stage with which it 
terminates in a complete example. 
All of these stages differ materially from each other as a rule, and it is 
often convenient to divide them into sub-stages, connoted by the prefixes ana., 
meta-, and para-. Thus the nepionic can be separated into ananepionic, meta- 
nepionic, and paranepionic, and it is often essential to treat the neanic and 
gerontic stages in the same manner.” 
1 [The following terminology, and the descriptions and arrangement of the groups from this 
point onward have been revised by Professor Alpheus Hyatt ; and certain alterations as well as 
much new matter have been introduced into the translation of the original text with the author's 
permission. —TRANS. | 
2 For a more extended discussion of terminology that can be advantageously used in descriptions 
of shells of this class see Hyatt, A., Phylogeny of an Acquired Characteristic (1894), p. 422 e¢ seq. 
. 
