481 
cuts at right angles to the long axis of the narrow pear-shaped 
umbilical perforation and that there is a faint but well-defined dor- 
sal furrow in the dorsum of the paranepionic volution. In Fig. 
15 a somewhat older section is shown and the siphuncle of the 
metanepionic substage is propioventran. The umbilical perforation 
is narrower in consequence of the approximation to the dorsum of 
the parenepionic whorl and the dorsal furrow is well-defined at 
this bend, and broader and deeper than it is beyond this in 
Hig. 16. 
The birth of the dorsal furrow is shown in Fig. 16, since one 
can see here the distinct outlines of the metanepionic volution 
broadening out internally, and the dorsal side of this remaining 
stiff and rounded while the plastic dorsal side of the growing 
paranepionic volution was bent into a dorsal furrow while being 
built around this abrupt bend. In Fig. 17 the section has passed 
inside of the paranepionic whorl, and the aspect begins to be 
confused by the fact that it cuts across the septa and shell. This 
and Figs. 18 and 19 are similarly confused, and are of value only for 
tracing the positions of the siphuncle. This organ obviously begins 
in a subventran position, becomes propioventran in the metane- 
pionic and paranepionic, centrodorsan in the ananeanic after the 
completion of the first volution, and finally subdorsan in the meta- 
neanic substage on the third volution. ‘This position, is retained 
throughout life, as is shown in the section, Fig. 13, Pl. vi. 
The innermost volution shown in the side view of the same speci- 
men (Fig. 12) is the last quarter of the third and first quarter of the 
fourth volutions. The smooth, still kidney-shaped whorl of the last 
quarter of the third volution in Fig. 13 shows the paraneanic sub- 
stage. The third sectional outline of a whorl below the central 
rounded ananepionic tip gives this age, and the third sectional 
outline of a whorl above the same ananepionic centre gives the sec- 
tion across the first quarter of the fourth whorl, which is the an- . 
ephebic stage, and has a very distinct outline. Owing to the de- 
crease in the rate of growth of the lateral diameters, the sides and 
abdomen have become contracted and the kidney shape of the 
earlier ages has been exchanged for a helmet shape in outline.* 
The living chamber in this specimen occupies at least the greater 
part of one-half of a volution, but its exact length could not be ascer- 
* This section is unluckily in a position which isthe reverse of that of Fig. 12. 
PROC. AMER. PHILOS. 80C. XXXII. 143. 31. PRINTED JUNE 80, 1894. 
