594 MOLLUSCA phylum vi 



of growth, on both the dorsum and venter, thus indicating that the young 

 animal did not possess a large hyponome. An orfhoceracone 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 depressum. 



Nautilicones 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 trorJioceracones, 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. NAUTILOIDBA Zittel. 



The conchs are camerated orthocones and cyrtocones in the young of primitive 

 forms, becoming cyrtoceracones like the adults of these same ancestral shells in the young of 

 more si^ecialised and coiled shells. Apertures have, as a rule, ventral or hyponomic 

 sinuses, and crests on the dorsum. Septa are concave along the mesal plane towards tlie 

 apex. Sutures straight or undulated, rarely with sub-angular lobes and saddles, and these 

 are probahly never acutely angular, as in the Ammonoidea. Each segment of the siph- 

 uncle is composed of a funnel and sheath as among primitive Ammonoids, hut 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 

 or cicatrix, ivhich is more or less compressed elliptical, never transversely elliptical or 

 depressed, and is sometimes hidden by the protoconch or its shrunken remnants. 



The order may be subdivided according to the general external features of 

 the shell and structure of the siphuncle into five sub-orders, as follows, named with 

 reference to peculiarities of the funnels : — Holochoanites, Mixochoanites, Schistochoanites, 



