405 
is to say, one with the transverse axis longer than the dorso-ventral 
and is apt to be ornamented with coarse ridges, whether the shell 
is subsequently smooth or remains ridged. The septum succeeding 
the first septum among nautiloids and also belonging to the meta- 
nepionic substage has a large siphuncle compared with the ventro- 
dorsal axis, and this has been called the ‘‘ macrosiphonula.’’ The 
remarkable observations of Henry Brooks have amply sustained 
these statements made in previous publications, as may be seen in 
diagram Fig. 11, Pl. i. 
The macrosiphonula brings before the observer certain internal 
characteristics which, although much altered, appear to have been 
derived from the earliest ancestors of the nautiloids, Diphragmo- 
ceras. ‘The metanepionic substage is therefore in part in all forms 
very primitive, in spite of the fact that in highly accelerated nau- 
tilian shells it is very much modified and also that some of its 
external characteristics are derived from the more recent ancestors 
of its own ordinal or subordinal phylum. 
The paranepionic substage begins with the third septum and its 
accompanying living chamber and, so far as I know, it does not 
carry any external characteristics derived from a very remote 
ancestry but usually in nautilian shells points very definitely to 
some known or unknown gyroceran ancestor. This is broadly 
shown in the fact that in the greater number of the more generalized 
forms of nautilian shells the three parts of the nepionic stage occur 
before the whorls touch. The external characteristics and form of 
the metanepionic and paranepionic substage have been largely 
derived from the immediate ancestors of the species. They often 
have their corresponding phyletic forms within their own genetic 
group or family, whereas the characteristics of the ananepionic 
substage are, in large part at least, derived from remote ancestors. 
Thus by the aid of direct observation it is not difficult to see 
that the substages of development in ontogeny are the bearers of 
distal ancestral characters in tnverse proportion and of proximal 
ancestral characters in direct proportion to their removal tn time 
and position from the protoconch or last embryonic substage. It is 
already generally admitted that this law is true of the embryonic 
stages themselves with reference to the protembryo, although most 
observers would hardly dare state this in the same positive terms as 
here employed because they are confused by what they call abbre- 
viated development. ‘They have not traced the systematic regu- 
