MOLLUSCA. 827 



Orthoceratites BREYN. Sliell elongato-conical, straight, poly- 

 thalamous, with septa perforated by siphon middle or subcentral. 

 (Straight, not spiral Nautili.) 



Sp. Orthoceratites regularis SCHLOTH., BRONN Leth. geogn. Tab. I. fig. 10, in 

 ThonschieferTcallc (WenlocJc shale?) 



Gen. Actinoceras BRONN does not, according to QUENSTEDT, differ from 

 Orthoceratites. 



Appendix to the Nautilacea. Family of Ammonitacea. Shell 

 polythalamous, with siphon external, marginal. Septa at the margin 

 lobate, sinuate. 



Cornu Ammonis. These petrifactions are of various, some of very re- 

 markable, size. If the animal was placed, as we may conclude from analogy, 

 within the shell as Nautilus is, the siphon would lie on the ventral side 

 towards the margin of the funnel, and the expression Sipho dorsalis, used 

 by Palaeontologists, is consequently incorrect. Ammonites are met with in 

 great numbers, especially in the transition and older secondary formations. 

 In the lowest chalk-strata they are still numerous, but afterwards decrease 

 rapidly and after the chalk-periods are met with no more on our earth. 



Compare G-. DE HAAN Specimen inaug. exhibens Monographiam Ammo- 

 niteorum et Goniatiteorum, L. B. 1825, 8vo; L. DE BUCH Note sur les 

 Ammonites, Ann. des Sc. not. xvn. 1829, pp. 267275; Sur la distribution 

 des Ammonites en families, Ann. des Sc. nat. xvm. 1829, pp. 417426; 

 Ueber Ammoniten, iiber ihre Sonderung in Familien, &c., Physik. Abhandl. 

 der Berliner ATcad. d. Wissensch., aus dem Jahre 1830; Ueber Ceratiten, 

 ibid., a. d. J. 1848; F. A. QUENSTEDT Die Cephalopoden, Nebst einem 

 Atlas von 36 Tafeln. Tubingen, 1849, 8vo. 



Ammonites BREYN. Shell spiral, discoidal. 



Sub-genera: Goniatites DE HAAN, Ceratites ejusd. and some 

 others seem only to form sections, with species passing from one 

 section into another. 



Sp. Ammonites nodosus BRUG., Ceratites nodosus DE HAAN, BRONN Leth. 

 geogn. Tab. XI. fig. 20 ; V. BUCH Ueb. Ceratiten, Tab. I. n. fig. i, from the 

 Muschelkalk. The species of the section of Goniatites are found in the 

 transition formations. 



Scaphites PARKINSON. 



Hamites SOWEREY. Shell incurved at the apex, produced into 

 a straight tube. 



Sp. Hamites rotundus SOWERBY, BRONN Leth. geogn. Tab. xxxm. fig. 9, from 

 the chalk-formations. 



Turrulites MONTF., LAM., Turrites DE HAAN. 



