530 



MOLLtJSCA 



SUB-KINGDOM VI 



FIG. 1083. 



Ooceras (Cyrt.) Baylei, 

 Barr. sp. Silurian (E); 

 Lochkow, Bohemia 

 (after Barrande). 



tube, or sometimes absent altogether. Living chamber short and like that of Cyrtorizo- 

 ceras ; aperture not infrequently sub-trigonal in outline, but always 

 open. 



Ooceras, Hyatt (Oonoceras, Hyatt), (Fig. 1083). Cyrtoceracones 

 more elongated and usually more compressed than in Cyrtorizoceras, 

 but otherwise similar except in structure of siphuncle. Septa 

 rise rapidly on ventral side, and may bend sharply orad, forming 

 a funnel ridge or shoulder on that side, but disappearing on. the 

 opposite side of the same funnel. When the funnel itself is 

 absent, the ridges look like reversed funnels or collars. Silurian. 



Cyrtoceras, Goldf. Large exogastric, breviconic cyrtoceracones ; 

 sections depressed elliptical or approximating to trigonal, the 

 dorsum more or less flat, and venter elevated. Aperture con- 

 tracted 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, nummuloidal, 

 with well -developed actiniform lamellae, and with an endo- 

 siphuncle in later stages of ontogeny. Devonian. 



Family 7. Oncoceratidae. 



A 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 gerontic stage. Siphuncle tubular or highly 

 nummuloidal, without deposits. 



Eremoceras, gen. nov. Cyrtoceracones similar to Cyrtorizoceras, but living chambers 

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

 loidal. Type E. (Cyrt.) syphax, Bill. sp. Ordovician. 



Cyclostomiceras, gen. nov. Slender, short, exogastric orthoceracones and cyrtocera- 

 cones, circular or compressed in section. Living chamber as compared with camera ted 

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

 gerontic stage. Type C. (Pot.) cassinense, Whitf. sp. Ordovician to Devonian. 



Oncoceras, Hall. Compressed exogastric cyrtoceracones with sections like Cyrtori- 

 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. 



Sub-genus : Meloceras (Melonoceras), Hyatt. Similar to the last, but lateral edges 

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



Family 8. Poterioceratidae. 



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 known, slightly 

 nummuloidal 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. 



Clinoceras, 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 differentiated from the hyponomic sinus, and contraction may take 

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



