SUB-CLASS I TETRABRANCHIATA—NAUTILOIDEA 5s 
The many different forms of Nautiloid shells may be grouped into a few 
leading types, as follows :—An orthocone is the young of the straight as well as 
many of the coiled forms. In this, although straight, the bands of growth 
are broader on the venter than on the dorsum, and there is no hyponomic 
sinus. <A cyrtocone is the similar stage which replaces or, as is oftener the 
case, succeeds this and is curved. Both of these may have crests in the bands 
of growth, on both the dorsum and venter, thus indicating that the young 
animal did not possess a large hyponome. An orthoceracone is the older stage 
of a straight form, and is nearly or quite straight on both venter and dorsum ; 
the bands of growth are approximately equal, but there is usually a hyponomic 
sinus. Cyrtoceracones are shells curved like Cyrtoceras on both venter and dorsum. 
Gyroceracones are curved in a loose spiral like Gyroceras, the volutions being 
sometimes in contact, but there is no impressed zone, i.e. the venter is not 
involved by the overgrowth of the dorsum belonging to the next outer whorl. 
The impressed zone in its primitive form is the longitudinal impression 
formed in the dorsum by the contact of the whorls. This is divisible into 
two kinds—the contact furrow, arising and lasting only when the whorls are 
in contact ; and the dorsal furrow, arising through inheritance in the young 
before the whorls come in contact. There is also a third modification, which 
for the present may be called the persistent dorsal furrow. This occurs in the 
free senile whorls of some shells, and is a remnant of the impressed zone. 
Finally, there is a furrow arising only from contact in the old age of some 
distorted Ammonoids, and hence may be called the gerontic contact furrow. 
Cyrtoceracones and gyroceracones do not usually have impressed zones, but 
an exception is furnished by Cyrtoceras depresswm. 
Nautticones are closely coiled shells having an impressed zone. This may 
be only a very slight contact furrow, or a hereditary dorsal furrow deepening 
by growth and involution, as in Nautilus. Torticones are asymmetrical spirals 
like those of a Gastropod, either loosely or closely coiled. These may or 
may not have impressed zones. Among Nautiloids they may be distinguished 
as trochoceracones, etc., according to their form, and among Ammonoids as 
turriliticones, etc., when more precise descriptive terms are required. A special 
nomenclature is employed in describing the position of the siphuncle, which 
is of convenience in technical treatises, but may be omitted here. The septal 
chambers have been termed camerae in the sequel, because this avoids any 
assertion with regard to their contents, such as is implied by ‘“air-chambers ” 
and the like. The less appropriate term ‘loculus ” has been used with the same 
meaning by Holm. 
Order 1. NAUTILOIDEA. 
The conchs are camerated orthocones and cyrtocones tn the young of primitive 
forms, becoming cyrtoceracones like the adults of these same ancestral shells in the young of 
more specialised and coiled shells. Apertures have, as a rule, ventral or hyponomie 
sinuses, and crests on the dorsum. Septa are concave along the mesal plane towards the 
apec. Sutures straight or undulated, rarely with sub-angular lobes and saddles, and these 
are probably never acutely angular, as in the Ammonoidea. Each segment of the siph- 
uncle is composed of funnel and sheath as among primitive Ammonoids, but the funnel 
persists throughout life in the ontogeny of all forms (except perhaps Nothoceras). Collars 
around the oral openings of the funnel are present in the later stages of Ascoceras (and 
Nothoceras?). Apex cup- or saucer-shaped, and marked by a circular or elongated cavity 
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