SUB-ORDER E CYRTOCHOANITES 529 
(?) Tretoceras, Salter. Orthoceracones having a centren nummuloidal siphuncle, 
similar to that of Actinoceras but smaller, and with a superficial tubular siphuncle 
(so-called), having very long but not Holochoanoidal funnels. Sheath not yet shown 
to be present. It is possible that the structure referred to is a peripheral pseudo- 
siphuncle formed by abnormal condition of the septa. Silurian. 
II. AcTINOSIPHONATA. 
Orthoceracones, cyrtoceracones, gyroceracones, and a few discoidal nautilicones. Shells 
frequently breviconic, in which case gerontic living chambers and apertures are more 
highly contracted than in all other Nautiloids. Siphuncle sometimes empty ; organic 
deposits when present in the form of laminae radiating from the sheath of each segment 
towards the intervor. These internal caleareous septa are united only in their peripheral 
parts, not meeting at the central axis so far as known, and also liable to be more or less 
interrupted in the transverse plane of each funnel. The interior is consequently an 
actiniform endosiphuncle with rays extending outwardly between the laminae of the 
deposits, 
Family 4. Jovellanidae. 
Orthoceracones and cyrtoceracones with slightly compressed oval, or depressed and 
more or less sub-trigonal sections. Shells smooth or partially annulated. Siphuncle large, 
with well-developed actiniform lamellae, and distinct endosiphuncles. Aperture open and 
living chamber uncontracted in gerontic stage. 
Includes Jovellania, Bayle; Triplewroceroceras, Hyatt. Silurian and Devonian. 
Mixosiphonoceras, gen. noy. Type M. (Cyrt.) desolatuwm, Barr. Silurian and Devonian. 
Projovellania, gen. nov. Type P. (Cyrt.) athleta, Barr, Silurian. 
Family 5. Rizoceratidae. 
Orthoceracones and cyrtoceracones expanding regularly by growth throughout life, the 
living chamber very slightly or not sensibly contracted in gerontic stage. Aperture con- 
stantly open, and with slight dorsal as well as somewhat deeper and 
broader hyponomic sinuses. Siphuncle generally small and empty, 
but actiniform lamellae and an endosiphuncle sometimes occur. Shells 
as a rule smooth or with transverse bands only, but longitudinal 
striae often present in earlier stages. 
Rizoceras, Hyatt (Fig. 1082). Orthoceracones and exo- or endo- 
gastric cyrtoceracones having circular or elliptical sections. Living 
chamber extraordinarily large and long as compared with camerated 
part. Silurian to Carboniferous. 
Cyrtorizoceras, gen. nov. Sections more compressed than in 
Rizoceras, living chamber shorter and apt to be more or less 
laterally compressed in gerontic stage, but the dorso - ventral 
diameters only very slightly so or not at all. Sutures more sinuous, 
and with decided ventral and dorsal saddles. Type C. (Cyrt.) 
minneapolis, Clarke sp. Ordovician and Silurian. 

Fia. 1082. 
Rizoceras robustuwm, 
= ; Barr. sp. Silurian (EB); 
Family 6. Ooceratidae. Butowitz, Bohemia. 
: Aperture open. 1/9. 
Orthoceracones and gyroceracones with closely set septa and large 
nummuloidal siphuncle in later stages of the ontogeny, but tubular in the young. 
Actiniform deposits oftener present than in Rizoceratidae, but not general. Funnels very 
variable, sometimes minutely plicated or hook-like in section, confined to dorsal side of 
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