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mic sinus, lateral crests and dorsal crests bending inwards and con- 

 tracting the opening. It is also of interest in this connection as 

 showing how narrow and comma-shaped the umbilical perforation 

 may be without affecting the form of the dorsum, and especially 

 with regard to the history of the degeneration of the impressed 

 zone on the dorsum of the free whorl and living chamber. 



Ophidioceras. 



Ophidioceras rudens. 



Ophidioceras barrande (Syst. Si'/., PL xlv) ; PL viii, Figs. 29-35. 

 Loc. Bohemia. 



This species has a flattened comma-shaped umbilical perforation 

 and, although the increase in size is rapid, it is not excessive in 

 the lateral diameters as compared with the ventro-dorsal from the 

 ananepionic substage to the paranepionic. The result is a volution 

 which curves evenly about the core of the perforation and preserves 

 the rounded dorsum and the general aspect of the section without 

 great modification throughout the nepionic stage. The cicatrix is 

 well-marked, as, shown in Fig. 30, and the ananepionic substage 

 has an elongated ventro-dorsal and short transverse diameter. 



In the metanepionic substage the whorl becomes broader on the 

 venter than on the dorsum, and in the paranepionic the dorsum 

 spreads, becoming broader, but does not quite equal the venter in 

 breadth. In the ananeanic substage the longitudinal ridges become 

 more prominent and more easily observable and the costations also 

 appear. 



The contact furrow begins as soon as contact is complete and is 

 at once deep and definitely defined, as a hollow fitting over the 

 ananepionic tip, and it completely covers in this substage. The 

 contact takes place on the dorsal side of the ananepionic substage 

 and the furrow is deeper at this point in proportion to the whorls 

 than it is at any subsequent age. 



There were two specimens showing the nepionic stages of this 

 species under observation, the one drawn and this one. Both 

 exhibit the peculiar globular form of the apex, and the well-defined 

 ana- and metanepionic substages, which can be quite closely com- 

 pared with those of Nautilus fiompilius, and they have similar con- 

 strictions to the first two constrictions depicted in Henry Brook's 

 drawings on PL i of this paper. 



