CLASS V 



CEPHALOPODA 611 



opposite side of the same fuunel. When the funnel itself is absent, the ridges look 

 like reversed funnels or collars. Silurian. 



Cyrtoccras Goldf. Large exogastric, breviconic cyrtoceracones ; sections depressed 

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

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

 an acute angle with the central axis, so that the dorsal side is very much shorter than 

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

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



Family 7. Oncoceratidae Hyatt. 



A phylogerontic group of breviconic orthoceracones and cyrtoceracones similar to 

 Gyrtorizoceras, but shells much shorter and living chamber Visually contracted, especially 

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

 nummuloidal, tvithout deposits. 



Ercmoceras Hyatt. Cyrtoceracones similar to Cijrtorizoceras, but living chambers 

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

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



Cyclostomiceras 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 Avith open aperture in 

 gerontic stage. Type C. (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. 



Sul>genus : 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 

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

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

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

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

 to 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 ; Acleistoceras Hyatt). Orthoceracones and 

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

 area not decidedly difl'erentiated from the hyponomic sinus, and contraction may take 

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



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

 aperture, 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 

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

 primitive forms. Even the latter usually have the brachial distinctly marled off from 

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



