﻿516 



chamber straight on the dorsum and producing a slight curvature 

 in reverse of the spiral on the venter. In Ophidioceras this is 

 accompanied by the outgrowth of a transverse dorsal spur which 

 divides this region into two distinct parts, as shown in the same 

 figure. The inner part of the living chamber, Figs. 32, 35, has the 

 central dorsal face and lateral dorsal faces derived from the closely 

 coiled whorls. These parts and the whole zone disappear as they 

 approach the dorsal spur, Figs. 32, 34. On the outer side of this spur 

 the impressed zone reappears, but it is the primitive form of this 

 which reappears and is perpetuated, the dorsal faces of the ephebic 

 impressed zone are not reconstructed, Figs. 32, ^^. The spur is not 

 a prolonged costation ; it occurs indifferently between two costations 

 or as the continuation of a costation, and is obviously independent 

 in its origin and construction. 



These facts show that there is some constantly recurring peculi- 

 arity in the growth of these shells which causes the outgrowth of the 

 dorsal spur, and this outgrowth temporarily interrupts the construc- 

 tion of the impressed zone. Notwithstanding this interruption, the 

 latter has even in the largest shells made such an impression on the 

 organism or become so fixed in the organization that, as soon as 

 the outgrowth stops, the impressed zone reappears. The spur 

 either directly obliterates the ephebic characters of this zone, the 

 dorsal faces, or else fills the space which transitional characters 

 would have occupied, so that when the zone comes in beyond the 

 spur it is evenly rounded as in the neanic stage. It is, however, 

 shallower and nearer the aperture it, in part or almost entirely, dis- 

 appears. The spur always occurs as a divide between the excentric 

 spiral and the reversed curve which begins beyond it, and has 

 some obvious connection with this change in the mode of building 

 the shell, as is shown in Fig. 29, PL viii. 



The sutures occupying the nepionic whorl are six in number and 

 very wide apart from the first to the fourth.* The fifth and sixth 

 show approximation and the seventh is about the normal distance. 

 The growth of the shell in the nepionic substages and in the ana- 

 neanic substage, to which the fourth, fifth and sixth sutures belong, 

 must have been very much more rapid than subsequently. They 



*The great size and depth of the apical air chamber is very remarkable. It is 

 not satisfactorily settled in my mind that there is not at least one septnm nearer the apex 

 than that which is here counted as the first, but even in "well-preserved specimens this 

 has not been observed. 



