286 



In the Xautiloidea, the sutures are, as a rule, simple or but slightly 

 lobed, and the siphuncle is commonly central or excentric, but seldom 

 marginal, with the funnels generally directed backwards. The embryonic 

 shell, or protoconch, is rarely retained. 



In the Ammonoidea, the sutures are lobate to highly complex, with for- 

 ward-bending saddles and backward-bending lobes, both often liighly 

 compound, usually with a mntrnl or siphonal lobe marking the position of 

 the marginal siphuncle. The siphonal funnels are commonly directed for- 

 wards. A globular or egg-shaped embryonic chamber, or protoconch, is 

 generally retained. The position of the hyponome variously influences 

 the form of the aperture and the character of the ornamentation in the 

 Ammonoids as well as in the Nautiloids. 



The shells of cephalopods are either straight (more or less conical) or 

 variously curved and coiled to close involution. 



Note. — The anatomy of modern cephalopods is treated of in text- 

 books of zoology. A detailed description of fossil genera will be found 

 in Hyatt's "Genera of Fossil Cephalopods" (Proc. Bost. Soc. Nat. Hist., 

 Vol. XXII., 1883). A detailed classification and s^-nopsis of families is 

 given in Hyatt's article on Cephalopods, in Zittel's "Handbook of 

 Palaeontology " (Eastman's translation). All the Ammonoids noted below 

 are described in great detail, and with especial reference to their stages of 

 development, in Clarke's " Naples Fauna" (16th Ann. Rep't N. Y. State 

 Geol., 1898). References to the Palaeontology of New York are made in 

 the text. 



NAUTILOIDEA. 

 Genus ORTHOCERAS. Brevxifs. 



[Ety. : 0/'</<os, straight; keras, hovn.l 

 (1732: Dissertatio Physica de Polythalamiis.) 



Shell a straifiht conical tube, with a large body chamber 

 and numerous air chambers, separated bv convex septa. 

 Sutures simple, at right angles to the long axis of the shell : 

 siphuncle central, sub-central, or excentric, cylindrical or 

 sometimes widening in the chambers. Surface smooth or 

 variously ornamented by transverse or longitudinal stria^, 

 or by annulations. 



Okthocekas EPviExsE. Hall. (Fig. 220.) ( Pal. X.'Y., Vol. 

 v., Pt. IL.p. 274, PI. XL.) 



I)istiniivishin<^- f'hnra,cters. — Large size; straight, robust 

 form, regularly enlarging to the slightly contracted ajDer- 

 ture; circular cross section; apical angle, eight degrees; 



