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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 

 Fig. 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. iS 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, PI. 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 iu a position which is the reverse of that of Fig. 12. 



