﻿439 



a very small umbilical perforation, the bending of the paranepionic 

 stage taking place with great abruptness. When seen laterally 

 (Fig. 4, PI. iv) the umbilicus shows a much larger perforation than 

 exists internally. This is due to the curvature of the perforation 

 and its decrease in diameter internally. Starting from either side, 

 it is an unsym metrical cone with a pear-shaped base, which de- 

 creases internally and bends in a bow-like curve as it crosses 

 between the dorsan surfaces of the meta- and paranepionic sub- 

 stages. The external orifices are usually owing to the fact that the 

 matrix is difficult to clean out, apparently broader than they really 

 are. The actual diameter is about i mm., diminishing to .5 mm. 

 at the center. In section, however, it may be seen, as in Fig. 9, 

 to have a minute perforation between the metanepionic dorsum at 

 the center below and the paranepionic dorsum just above this. The 

 outline of the section (Fig. 10) probably passed not far from the 

 apex in this specimen, probably through the metanepionic substage, 

 judging from the outline of the section (Fig. 9, and the enlarged out- 

 line, Fig. 11), which shows transitional characteristics from the 

 dorsoventrally elongated oval of the whorl of the ananepionic sub- 

 stage common to most Nautiloids and the transverse oval of the 

 earlier paranepionic also found at this substage in a large number, of 

 nautilian shells. This outline, Fig. ir, is similar to that of the 

 shells of Nautilus pompilius at the same age. 



The paranepionic substage of Fig. 9 has also an outline simi- 

 lar to that of Nautilus pompilius at the same age, being kid- 

 ney-shaped, with a broad but well-defined dorsal furrow. The 

 presence of this dorsal furrow, although the whorls do not touch, 

 appears at first to justify the opinion that this is a case in which 

 inheritance may be assumed. The paranepionic dorsum is, how- 

 ever, very closely approximated to the dorsum of the metane- 

 pionic substage, and it seems possible that this proximity modified 

 the shape of the secreting edge of the dorsal side of the mantle 

 and caused the corresponding impression shown in the shell. At 

 any rate, it is not safe to assume that this represents any hereditary 

 tendency. The exceedingly quick growth from the apex to the 

 paranepionic and the sudden curvature of the early paranepionic 

 whorls might have produced this also, as pointed out in other 

 similar cases. In making another specimen of this species (Fig. 4, 

 PI. iv) I was fortunate enough to crack the fossil so as to expose the 

 entire length of the cast of the umbilical perforation. I found this 



