SUB-ORDER E CYRTOCHOAN1TES :. - 



(!) Tretoceras, Salter. Orthoceracones having a n-ntivn nummiiloidal 

 similar to that of Actinocera* but smaller, and with a >njn-rtii-.ial tubular .-i|.liiiin-|,- 

 (so-called), having very long but not Holochoanoidal funnels. Sheath not yet 

 to be present. It is possible that the struct un- ivirrn-d to is a peripheral 

 siplmucle formed by abnormal condition of tin- septa. Silurian. 



II. ACTINOSIPHONATA. 



Orthoceracones, cyrtoceracones, gyroceraconcs, and a few discoidal //,</, //.//Y/'/v*//. . ,s'// II* 

 fret//" ntly breviconic, in which case gerontic living chambers and apertures an wor< 

 highly contracted flnm in nil other Nautiloids. Siphuncle sometimes empty; organic 

 deposits when present in the form of laminae radiating from the sheath of each segment 

 towards the interior. These internal calcareous 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 en<l*ii>h uncle with rays extending outwardly between the laminae of the 

 deposit*. 



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 ; 2 J ripleuroceroceras, Hyatt. Silurian and Devonian. 

 Mixosiphonoceras, gen. nov. Type M. (Cyrt.) desolatum, 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. 



r 10. 1082. 

 Rizoceras 



Family 6. Ooceratidae. SSwSt S 



Aperture open. 

 Orthoceracones and gyroceracones with closely set septa and large 



nummuloidal siphuncle in later stages of the ontogeny, but tubular in tl<> 

 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 

 VOL. I 2 M 



