the genus Cryptoccras in Silurian Rocks, 115 



1. GoMPHOCERATiDii': : — Apei'tui'e contracted. Gomphoceras 

 (including HalVsOrt/wcerasfusiforme); Phragmoceras ; Oncoceras; 

 Lituites (?). 



2. HETEROSiPHONiDiE : — Apei'tuvc unknown, perhaps con- 

 tracted. Siphuncle more or less complicated, or otherwise mar- 

 ginal, with conical Orthoceras-like shell. Septa simple or slightly- 

 wavy. (See remarks below.) Endoceras ; Cameroceras (?) ; Go- 

 nioceras ; Ormoceras ; Ascoceras. 



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

 central or sub-central. Orthoceras ; Nautilus; Lituites i^.) ; 

 Hortolus; Aploceras (including HalPs Ci/r^06-er«5 «wwi^/«^Mm ?) ; 

 Nautiloceras ; Trochoceras, 



4. Trocholitidj]: : — Aperture open. Septa simple. Siphuncle 

 internal or "ventral." Trocholites, 



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

 phuncle external or " dorsal." Cyrtoceras ; Gyroceras ; Crypto- 

 ceras. 



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

 internal. Clymenia ; Subclymenia. 



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

 internal or nearly so, and very large. Aturia [Megasiphonia] — 

 a Tertiary form. 



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

 Siphuncle external. Goniatites {Aganides) ; Bactrites. 



9. CeratidyE : — Aperture open. Septa with denticulated 

 lobes. Siphuncle external, Ceratites ; Baculina, 



10. Ammonitid^ : — Aperture open. Septa foliated. Si- 

 phuncle external. Ammonites; Crioceras ; Scaphites ; Ancrjlo- 

 ceras ; Toccoceras *? ; Hamites ; Ptychoceras; Baculites; Turrilites ; 

 Helicoceras ; Heteroceras. 



Under the name of Heterosiphonidse, we have separated from 

 the Nautilidse, all of those more or less imperfectly known 

 forms (commonly classed with Orthoceras) 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 

 Ormoceras, for example, we feel justified in holding to the sepa- 



the respective families become ill-defined, and the appreciation 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 

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

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

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

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



8* 



