608 MOLLUSCA phylüm vi 



Suborder E. CYRTOCHOANITES Hyatt. 



Shells varying from orthoceracones to nautilicones, none of them highly ornamented, 

 although some are annulated or costated, and in rare cases slightly nodose. Sutures as a 

 rule simpler than in the Orthochoanites. Siphuncle varies exceedingly, passing from tubulär 

 in the young, and even in the full-grown of primitive forms, to highly nummuloidal in the 

 adults of specialised genera, or again in some groups retaining constantly its primitive 

 character. The funnels, however, are as a rule hent outward or crumpled, and generally 

 short. 



I. Annulosiphonata Hyatt. 



Mostly orthoceracones and cyrtoceracones, with a few gyroceracones and very rarely 

 nautiliconeSj the last-named heing invariably discoidal. Apertures constantly open. 

 Siphuncle may he empty, hut organic deposits when present always gathered about or 

 encrusting the funnels as hollow or solid internal rings. Deposits sometimes sujfflcient to 

 form more or less annulated endosiphuncles, the rings heing opposite the camerae, alter- 

 nating with the septa, and extending outwardly. 



Family 1. Loxoceratidae Hyatt. 



Smooth orthoceracones and cyrtoceracones similar to the Orthoceratidae^ hut siphuncle 

 distinctly nummuloidal, and funnels very short and crumpled. Deposits not uncommon, 

 hut irregulär, and only irregulär endosiphuncles occasionally formed. 



Loxoceras M'Coy {Sactoceras Hyatt). Mostly orthoceracones, circiilar or elliptical 

 in section. Siphuncle supposed to be tubulär in the young, but highly nummuloidal 

 in later stages, centren or near the centre. Septa invariably single, and camerae 

 empty. Ordovician to Carboniferous. 



Campyloceras M'Coy {Aploceras d'Orb.). Breviconic cyrtoceracones or orthocera- 

 cones with smooth or finely ridged shells, circular or depressed elliptical in section. 

 Siphuncle centren or ventrad of centre. Carboniferous. 



Family 2. Uranoceratidae Hyatt. 



Cyrtoceracones, gyroceracones and nautilicones, with stout volutions. Siphuncle in 

 primitive forms highly nummuloidal, hut invariably empty ; in nautilicones it has less 

 nummuloidal segments, and is uniformly ventrad of centre, hut not near the venter. 

 Sutures with ventral saddles, lateral lohes, and also dorsal saddles in primitive forms as 

 well as the young of all shells. Ventral and dorsal lobes arise suhsequently in the 

 ontogeny of nautilicones. 



Uranoceras Hyatt. Stout, more or less breviconic cyrtoceracones, compressed 

 elliptical or sub-quadrangular in section. Sutures with broad ventral saddles, lateral 

 and dorsal lobes. Siphuncle large, nummuloidal, centren or ventrad of centre. 

 Devonian and Carboniferous. 



Gigantoceras Hyatt. Gyroceracones similar to the preceding, but having longer 

 living Chambers and more compressed volutions. Includes the largest known Nauti- 

 loid Shells. Type G. (Gyroceras) inelegans (Meek). Silurian. 



Family 3. Actinoceratidae Saemann. 



Orthoceracones and cyrtoceracones with siphuncle more or less ßlled hy rings of 

 organic deposits, and having an endosiphuncle in the central axis. Camerae may he 

 empty or filled to various degrees with organic deposits, even to the extent of solidifying 

 the entire shell previous to the gerontic stage. Shells smooth or annulated, but not longi- 

 tudinally ridged, at least in the latter stages. 



