CLASS V CEPH ALOPOD A 6 1 1 



opposite side of tlie saiiie funiiel. When the funnel itself is absent, the ridges look 

 like reversed funnels or collars. Silurian. 



Gyrtoceras Goldf. Large exogastric, breviconic cyrtoceracones ; sections depressed 

 elliptical or approximating to trigonal, the dorsum more or less flat, and venter 

 elevated. Apertiire contracted in gerontic stage to a T-shaped opening, and placed at 

 an acute angle witli the central axis, so that the dorsal side is very miicli shorter than 

 the ventral. Siphuncle large, nummuloidal, with well-developed actiniform lamellae, 

 and with an endosiphuncle in later stages of ontogeny. Devonian. 



Family 7. Oncoceratidae Hyatt. 



Ä phylogerontic group of breviconic orthoceracones and cyrtoceracones similar to 

 Cyrtorizoceras, but shells much shorter and living Chamber usually contracted, especially 

 in their transverse diameters during the gerontic stage. Siphuncle tubulär or highly 

 nummuloidal, without deposits. 



Eremoceras Hyatt. Cyrtoceracones similar to Gyrtorizoceras, but living Chambers 

 longer, and aperture more or less flaring and open. Siphuncle more or less nummu- 

 loidal. Type E. (Gyrtoceras) syphax (Bill.). Ordovician. 



Gyclostomiceras Hyatt. Slender, short, exogastric orthoceracones and cyrtocera- 

 cones, circular or compressed in section. Living Chamber as compared with camerated 

 part longer and larger than in most forms, less contracted, and with open aperture in 

 gerontic stage. Type G. {Gomphoceras) cassinense (Whitf.). Ordovician to Devonian. 



Oncoceras Hall. Compressed exogastric cyrtoceracones with sections like Gyrtori- 

 zoceras, but Shells as a rule much shorter and smaller, and siphuncle more distinctly 

 nummuloidal. Living Chamber also more flattened laterally, the aperture elongated 

 and often sub-trigonal, but typically open. Ordovician. 



Subgenus : Meloceras {Melonoceras) Hyatt. Similar to the last, but lateral edges 

 of the gerontic aperture grow inwards, and form pear-shaped outlines. Silurian. 



Family 8. Poterioceratidae Foord. 



Smooth, breviconic orthoceracones and cyrtoceracones having circular or depressed 

 elliptical sections. Gerontic aperture, except in primitive forms, is contracted and apt to 

 issume a sub-trigonal outline; it is laterally narrowed and approximates those of the 

 lext family only in Streptoceras. Outlines of aperture entire ; sutures straight or only 

 kightly sinuous. Siphuncle in this and remaining families, so far as knoum, slightly 

 mmmuloidal and empty in the young, but becomes larger ; in specialised forms it is apt 

 be more or less filled with radiating lamellae, and in late stages has an endosiphuncle. 



Glinoceras Mascke. Ordovician to Devonian. Sycoceras Pictet. Devonian. 



Poterioceras M'Coy (Apioceras Fischer ; Äcleistoceras Hyatt). Orthoceracones and 



exogastric cyrtoceracones, short and stout, with sub-trigonal gerontic aperture. Brachial 



krea not decidedly differentiated from the hyponomic sinus, and contraction may take 



)lace in all diameters or more extensively in the lateral. Ordovician to Carboniferous. 



Streptoceras Bill. Like the last but more arcuate, with laterally contracted 

 iperture, and a short hyponomic sinus distinct from the brachial area. Silurian. 



Family 9. Trimeroceratidae Hyatt. 



Smooth breviconic orthoceracones and cyrtoceracones similar to Poterioceras in aspect 

 md sutures, but more slender, eqjecially in the young, and aperture very distinct in 

 imitive forms. Even the latter usually have the brachial distinctly marked of from 

 fhyponomic area by ingrowth of sides of the aperture, and in all specialised shells the 



