530 MOLLUSCA SUB-KINGDOM VI 
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 
Fic. 1083. acute angle with the central axis, so that the dorsal side is very 
Ooceras (Cyrt.) Baylei, much shorter than the ventral. Siphuncle large, nummuloidal, 
SNe See aioe with well-developed actiniform lamellae, and with an endo- 
(after Barrande). siphuncle in later stages of ontogeny. Devonian. 








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Family 7. Oncoceratidae. 
A phylogerontic group of breviconie 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 £. (Cyrt.) syphax, Bill. sp. Ordovician. 
Cyclostomiceras, gen. nov. Slender, short, exogastric orthoceracones and eyrtocera- 
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 C. (Pot.) cassinense, Whitf. sp. Ordovician to Devonian. 
Oneoceras, 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, breviconie 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 
neat 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 endosiphwnele. 
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. 
