582 
materials. Gabb states that both the second and third arms en- 
velop more or less the preceding, and they must therefore have 
contact furrows in both stages. 
VI. SUMMARY. 
The importance of the impressed zone can be made apparent 
better by discussing the correlative facts of the morphology than by 
any other means. 
When one considers the mode of growth of the young of any 
one of the straight or primitive arcuate forms of Nautiloids, the 
prominent fact is the bilateral symmetry of the cone and the 
asymmetry of the ventral and dorsal sides as in Spyroceras (2) cro- 
talum, Figs. 10-12, p. 361, and the young of other forms, p. 360. 
It is obvious from these drawings and other observations that this 
asymmetry is due to the more rapid growth of the ventral as con- 
trasted with the dorsal side. This is shown by the greater breadth 
of the bands of growth and the intervals between the sutures that 
are greater on that side. Subsequently in the ontogeny of the 
straight. forms, in Endoceras, Orthoceras, the growth becomes 
more nearly equal and in many forms is practically equal and the 
shell is built out in nearly straight lines. The angles of the curves 
made by the dorsal and ventral sides near the apex are on this 
account entirely distinct from each other, the venter departing 
from the end of the cicatrix at a much wider angle than the dor- 
sum, which is much less inclined and soon tends to assume an 
almost straight line. 
The two sides, venter and dorsum, tend therefore to become less 
divergent after the nepionic stage is passed, but they nevertheless 
continue as long as the cone increases in the ventro-dorsal diame- 
ters to grow in more or less divergent directions during the neanic 
and ephebic stages except in the living chambers of certain species. 
In these the diameters become shortened towards the aperture and 
the sides converge more or less either in the lateral or ventro-dor- 
sal diameters or in all diameters. ‘This occurs in some species 
only in the gerontic stage, but in others it may occur at any stage 
after the nepionic. 
In all shell-covered cephalopods, so far as known, the nepionic 
shells have open apertures and all four sides are continually diverg- 
ent in these younger substages of development. ‘The asymmetry of 
