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The dorsum of the whorl becomes at the same time broader and 

 the whorl alters in shape to an approximately kidney-shaped outline 

 with the ventro-dorsal, shorter than the transverse diameter. After 

 this age the increase of growth proceeds more slowly. In the meta- 

 neanic substage, the costae and longitudinal ridges become well 

 developed, but the venter remains rounded and the lines of growth 

 show a deep, broad hyponomic sinus and lateral crests, and the 

 aperture at this stage must have been very distinct from that of the 

 next substage. 



In the paraneanic substage the central ventral zone appears at first 

 as a broad band, in low relief, arising obviously from the elevated 

 edges of the narrow hyponomic sinus, which begins to appear at 

 this age. In the anephebic stage, at the beginning of the third 

 volution, this acquires its specific prominence and characters. The 

 metephebic stage is introduced by the subsequent moulding of the 

 dorsum over this broad carination which modifies the outline of the 

 contact furrow in section, and gives it the peculiar central dorsal 

 face and narrow lateral dorsal faces as peculiar to this genus as are 

 the ventral modifications which give rise to them.* 



The sutures do not seem to be much modified after the nepionic 

 stage is passed by. The caecum, if a spot observed on the broken 

 apex of one specimen is correctly translated, is subventran or nearly 

 so in the first or metanepionic septum and the siphuncle is about 

 the same position relatively or propioventran in the paranepionic 

 substage as observed in two specimens and given in one of these, 

 Fig. 30, PI. viii, and then changes slowly to centroventran in the 

 anephebic substage. The living chamber is very long, being, if the 

 excentric free part were applied to the coil, almost one volution in 

 length. It is, as has been described by Barrande, present in small 

 (young?) shells, but I doubt its existence, as well as that of the pecu- 

 liar ophidioceran aperture, before the substage in which the ventral 

 zone appears. 



The free whorl in this genus is specially interesting, because even 

 in large shells the impressed zone is preserved on the dorsum in a 

 very significant way. It is well known that most of the shells, if 

 not all of this genus, have the lituitean bend, that is to say, the free 

 living chamber, after it becomes free and excentric, bends suddenly 

 ventrally, as in Fig. 27, PI. viii, making the last part of the living 



* This is usually called a keel or carina, but it is a modification of a different kind and 

 sometimes has keels upon its borders. 



