THE CEPHALOPODA 159 



GENERAL CHARACTERS OF AMMONOIDS. 



The body-chamber of the typical ammonoids varies 

 from half a whorl to over two whorls in length. The 

 aperture of nearly all Devonian forms has a hyponomic 

 sinus on the periphery, but in the Carboniferous period 

 some of the goniatites lose this, and begin to develop a 

 slight forward projection (rostrum) in its place. In the 

 Triassic and early Jurassic periods this rostrum becomes 

 more and more prominent, until in some cases it projects 

 as a narrow rod with a length greater than the height of 

 the whorl (Fig. 45, c-f). Sometimes the rostrum may 



FIG. 46. APERTURAL MARGINS OF CEPHALOPODA (continued). 



rostrum and lappets are excluded. (After Haug.) b, Hildoceras sp. , 

 Whitbian (Upper Lias). Rostrum, lappet, and radial line. (After 

 Wright.) c, Lucya cavata S. Buckman, Aalenian (concava zone), 

 Bradford Abbas (Dorset). . Constriction behind aperture. (After 

 Buckman.) d, Oppelia fusca (Quenstedt), Vesulian, CEschingen 

 (Wurttemberg. j Lappet constricted at base ; relation of radial line to 

 apenural margin well shown. (After Quenstedt.) e, e', Fontannesia 

 cutvata S. Buckman, Bajocian (discites zone), Bradford AbbdS 

 (Dorset). Very feeble rostrum, constricted (spoon-like) lappets. 

 (After Buckman.) f,f, Witchellia deltafalcata (Quenstedt), Bajocian. 

 Rostrum almost lost, lappets bent inwards to constrict aperture. 

 (After Quenstedt.) g, Cosmoceraselizabeth.ee (Pratt), Callovian (Lower 

 Oxford Clay), Christian Malford (Wilts). Long lappets, no rostrum ; 

 relation of radial line to apertural margin. (After Quenstedt.) h, h' , 

 Ofoites sp. , Bajocian, Lauffen (Wurttemberg). Incurved lappets, no 

 rostrum ; no evidence of lappets in radial line. (After Quenstedt.) 

 i, i' , Morphoceras pseudo-anceps (Ebray), Vesulian, St. Honore'-les- 

 bains (Nievre, France). (X. ) Lappets and bifurcated rostrum 

 uniting to divide aperture into five orifices A, median hyponomic 

 orifice; B, paired opening, probably for eyes; C, paired opening for 

 arms. (After H. Douvilld) j, f, Siemiradzkia comptoni (Pratt), 

 Callovian. Long incurved lappets, no rostrum, a constriction shown 

 some distance behind aperture ; radial line parallel to constriction, 

 not to apertural margin. (After Quenstedt.) k, k', Sph&roceras 

 brongniarti (J. Sowerby), Bajocian, Spherocone, aperture depressed, 

 without rostrum or lappets. (After Quenstedt. ) /, /', Bacvlites (Cyrto- 

 chilus) baculoides (Mantell), Cenomanian, Chardstock (Somerset). 

 (After Criik. ) All x , except where otherwise stated. A", rostrum ; 

 L, lappet ; S, constriction. 



