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, 33. 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 1s 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 septum nearer the apex 
than that which is here counted as the first, but even in well-preserved specimens this 
has not been observed. 
