BDB-OLASS i TETRABRANCHIATA-NAUTILOIDEA 



The many different forms of N;iutiloil shells may In- grouped into . 

 lending types, as follows : An orllini'nni- is tin; youm: of the straight a- \\vll as 

 many of the coiled forms. In this, although Mraight, the bands of growth 

 are broader on the venter than on the dorsuni, and there i> no hyponomic 

 sinus. A cyrtocone is the similar stagr which replaces or, as is oftener the 

 case, succeeds this and is curved. Both of these may have crests in tin; hands 

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

 animal did not possess a large hyponome. An ///y//ofv/v//-i,/,/< is the older stage 

 of a straight form, and is nearly or quite straight on both vmirr and dorsum ; 

 the bands of growth are approximately equal, but there is usually a hyponomic 

 sinus. Cyrtoceracones are shells curved like ( '////Wv /,/.-> on both venter and dorsum. 

 Gyroceracones are curved in a loose spiral like '/'///</'vras, 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 Cyrtocems depressum. 



X'intilicones 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 trochocemcones, etc., according to their form, and among Ammonoids as 

 tiirriliticones, etc., when more precise descriptive terms are required. A special 

 nomenclature is employed in describing the position of the siphuncle, w.hich 

 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. 



Tin' cnnclix in-'' camerated orthocon^ <nnl < ij rtocones in the young of jn-imftirf 

 forms, becoming cyrtoceracones lib- tin- ml nits of these same ancestral x//i7/x i,, tl,- ,/,/ ,,f 

 more specialised and coiled shells. Apert n n-x li<n-'\ <t* rnl>; n-iifrnl or A//y>/<,,//V 

 ^i a uses, and crests on the dorsum. Septa are concave alumi tin- ///<.-/ /il<ni> /c-,/ ///> fit, 

 "/"'' Sutures straight or undulat'-il, run I;/ n-ii/, xiih-i,<!nl<ir I <>!>,>.< <nn/ .</*//>, mxl these 

 are probably never acutely angular, as in flu' Ammonoidea. Each >////,/// ,if flu- .tijili- 

 uncle is composed of funnel and sheath as among primitii-r Anunnnniilx, hut th> fun ii' I 

 persists throughout life in the ontogeny of all forms (except perhaps Nothoceras). 

 around the oral openings of the funnel are present in the later stages of Ascoceras 

 Nothoceras?}. Apex cup- or saucer-shaped, and marked by a circular or i'l<>,i,/,if,,l <-,, n't,/ 

 VOL. I 2 L 



