the fjemiH Cryjitocoras in Silurian Racks. 115 



1. Go.viriiocKRATTD.T': : — Aperture contracted. GompJioceras 

 (including \[-d\V^(}rtlwccrns J'asiforme) ; Phr(i(jinoceras; Oncoccras ; 

 Lituites (t). 



2. Heteuosiphonid.e : — Aperture uiilvnown, perhaps con-^ 

 tractcd. Siphuncle more or less comj)licatcd, or otherwise mar- 

 ginal, with conical Orthoceras-like shell. Sej)ta simple or slightly 

 wavy. (See remarks below.) Endoceras ; Camcroco'as {J) ; Go- 

 nioccras ; Orrnoccras ; Aacoceras. 



3. Nautilid.e : — Aperture open. Septa simple. Siphuncle 

 central or sub-central. Ortlwceras ; Nautilus; Lituites {'■) ; 

 Hortulns ; Aplocfras (including Hall's Cyrtoceras annulutuni'i) ; 

 Nautiloceras ; Troctwceras. 



4. Tkocholitid.e : — Aperture open. Septa simple. Siphuncle 

 internal or "ventral.'" Trocholites. 



5. Cyrtoceratid.e : — Aperture open. Septa simple. Si- 

 phuncle external or " dorsal/' Cyrtoceras ; Gyroceras ; Crypto- 

 ceras. 



6. Clymenid.t: : — Aperture open. Septa lobed. Siphuncle 

 internal. Clymenia ; Suhclymenia. 



7. Aturid^ : — Aperture open. Septa lobed. Siphuncle 

 internal or nearly so, and very large. Aturia [Megasijihonia) — 

 a Tertiary form. 



8. GoNiATiD.^: — Aperture open. Septa with angular lobes. 

 Siphuncle external. Goniatiles {Aganides) ; Bactriies. 



9. Ceratid.e : — Aperture o])en. Septa with denticulated 

 lobes. Siphuncle external. Ceratites ; Baculina. 



K). Ammoxitid.e : — Aperture open. Septa foliated. Si- 

 phuncle external. Ammonites; Crioceras ; Scaphites ; Ancylo- 

 ceras ; Toxoceras'i; Hmnites ; Ptychoceras; Baculites; Turrilites ; 

 Helicoceras ; Hcteroceras. 



Under the name of Heterosiphonidfe, we have separated from 

 the Nautilidse, all of those more or less imperfectly known 

 forms (commonly classed with Ortlwceras) which possess a large 

 complicated 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 

 Ormocei'as, for example, we feel justified in holding to the sepa- 



the respective families become ill-defined, and the a])])reciation of trans- 

 ition groups much weakened ; whilst, at the same time, a necessity is occa- 

 sioned for the creation of sub-families or tribes. A classification which 

 does not show upon its face a greater distinction between Goniatites, Cera- 

 tites, and Ammonites, than between the last-named genus and Hamites or 

 Baculifes, for example, assuredly has no claim to be considered a natural 

 grouping. In the arrangement given in the text, the second family is 

 merelj- a provisional one, rendered necessary by our still imperfect know- 

 ledge of its included forms. — E. J. C. 



8* 



