608 MOLLUSCA phylum 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. Sittures as a 

 rule simpler than in the Orthochoanites. Siphnncle varies exceedingly, passing from tubular 

 in the young, and even in the full-groion of immitive 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 bent outward or crximpled, and generally 

 short. 



I. Anndlosiphonata Hyatt. 



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

 natitilicones, the last-named being invariably discoidal. Apertures constantly open. 

 Siphuncle may be empty, but organic deposits when present always gathered about or 

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

 form more or less annulated endosiphuncles, the rings being optposite the camerae, alter- 

 nating with the septa, and extending outivardly. 



Family 1. Loxoceratidae Hyatt. 



Smooth orthoceracones and cyrtoceracones similar to the Orthoceratidae, but siphuncle 

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

 but irregular, and only irregular endosiphuncles occasionally formed. 



Loxoceras M'Coy {Sactoceras Hyatt). Mostly ortlioceracones, circular or ellijstical 

 in section. Siphuncle supjiosed to be tubular in tbe 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. 

 Sij)huncle centi'en or ventrad of centre. Carboniferous. 



Family 2. Uranoceratidae Hyatt. 



Cyrtoceracones, gyroceracones and na%itilicones, with stout volutions. Siphuncle in 

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

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

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

 toell as the young of all shells. Ventral and dorsal lobes arise .subsequently in the 

 ontogeny of nautilicones. 



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

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

 and dorsal lobes. Siphuncle large, nununuloidal, 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 filled by rings of 

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

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

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

 tudinally ridged, at least in the latter stages. 



