266 ON THE OCCURRENCE OE THE GENUS 



7. Aturidjs : — Aperture open. Septa lobed. Siphuncle internal 

 or nearly so, and very large. Aturia (Megasiphonia)—~a Tertiary 

 form. 



8. Goniatid.2e : — Aperture open. Septa with angular lobes. 

 Siphuncle external. Goniatites (dganides ;) Bactrites. 



9. Ceratid^; : — Aperture open. Septa with denticulated lobes. 

 Siphuncle external. Ceratites ; Baculina, 



10. Ammonitid^e : — Aperture open. Septa foliated. Siphuncle 

 external. Ammonites; Crioceras ; Scaphites ; Ancyloceras ; Toxoceras? 

 Samites ; Ptycoceras ; Baculites ; Turrilites ; Hellicoceras ; Hetero- 

 ceras. 



Under the name of Heterosiphonid^i, we have separated from 

 the Nautiltd^i, all of those more or less imperfectly known forms 

 (commonly classed with Orthoceras) which possess a large compli- 

 cated siphon, or in which with other related characters, the siphon 

 is marginal. "We are fully aware that many objections may be urged 

 against this view, but until a true nautilus be discovered with the 

 peculiar character of siphuncle exhibited by Ormoceras for example, 

 we feel justified in holding to the separation of this latter form, with 

 Endoceras, &c, from the normal Orthoceratites. The external ridges 

 on the siphuncle of Endoceras, although so distinctly pointed out 

 by Hall, appear to be forgotten altogether in the descriptions of many 

 European palaeontologists. Ormoceras, notwithstanding the central 

 position of its siphuncle, is evidently closely related to Gfonioceras ; 

 and through that genus, though less closely, to Endoceras. 



If the separation of the Groniatites and Ceratites from the 

 Ammonitid-E be disapproved of, they may be placed in that family as 

 separate tribes. Our present object, however, is not to discuss the 

 classification of the chambered cephalopods, but to point out the 

 occurrence in our Silurian rocks of a type hitherto unannounced 

 below the Devonian formation. 



In the fifth of the above families, that of the Cyrtoceratid,e, 

 characterised by the presence of simple septa with external or so- 

 called " dorsal" siphuncle, we have three genera : Gyrtoceras, a 

 simply "horned" form, exceedingly abundant; Qyroceras, a discoidal 

 or u rolled-up" cyrtoceras, but without contiguous volutions ; and 

 Cryptoceras, likewise a discoidal form, but with contiguous whorls. 

 Of the last named genus, founded by D'Orbiont, but two species 

 appear to have been hitherto recognised : the C. sultuberculatus 

 (Nautilus sultuberculatus) from the Devonian beds of Nassau ; and 

 the C dorsalis (Nautilus dorsalis Phil.) from the carboniferous 



