CEPHALOPODOUS MOLLUSCA. 



251 



found in the tertiary strata. We give a figure (168 and 169) 

 of Megasiphonia zic-zac, from the London clay of Primrose 

 Hill. 



FIG 167. Nautilus Elegans. Chalk . 



3rd Family. AMMOISTITID^;. Siphon placed at the exter- 

 nal dorsal part of the septa ; shell spiral or straight, arched 

 or bent in various forms. The aperture generally prominent 

 at the external dorsal margin ; septa almost always angular, 

 or ramified at the borders divided into lobes and saddles. 



FIGS. 168 and 169. Megasiphonia zic-zac. 



The genera oncoceras, cyrtoceras, gyoceras, cryptoceras, 

 stenoceras, are found in the palaeozoic strata. Goniatites are 

 Devonian and carboniferous, and ceratites are triasic forms. 



G. AMMONITES.* Shell forms a regular spiral, rolled on 

 the same plane, with the turns contiguous. (Figs. 170, 171, 

 172.) 



D'Orbigny, after making due allowance for varieties 



* Pict. Atlas, PI. Ix. and Ixi. 



