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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. n, 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 inverse propo7'tion and of proximal 

 ancestral characters in direct proportion to their removal in 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- 



