598 



MOLLUSCA 



PHYLUM VI 



known only in their later stages of development. Sipliuncle of moderate size, sub- 

 ventran. Funnels reaching lialf-way across each camera, steeply inclined orad, and 

 split on tlie outer side. Closure of the walls effected by a 

 plate extending from the apical opening of each funnel through 

 the funnel itself orad to the apical opening of the next 

 beyond, and projecting into the interior as a flattened fold, 

 which is incomplete or open along the central axis. These 

 internal coUars or flat semiconical rings have been described 

 as complete cones (Dwight). Ordovician. 



Suborder D. ORTHOCHOANITES Hyatt. 



' Gerontic stages have uncontracted volutions and open aper- 



tures, except in a few uncoiled phylogerontic genera. Siphun- 

 Fio, 1108. cular Segments may he slightly nummuloidal, fusiform or tubulär, 



Conoceras pracpost^rum hut are never marJcedly nummuloidal, nor are the funnels hent 

 D)rVosä^^Bohemia^fafter ^harply outwards as in Gyrtochoanites. Deposits formed only in 

 Barrande). the siphuncles of Orthoceratidae and Kionoceras, and in them 



they are irregulär and no endosiphuncles occur ; other genera 

 have empty siphuncles. Funnels, as a rule, hoth longer and straighter than in Gyrto- 

 choanites, and in Aturia almost equal to those of Holochoanites. 



This group includes the greater number of Nautiloid forms, passing from the 

 smoothest to the most highly ornamented of Paleozoic shells, continuing in the Trias 

 as nautilicones of complex ornamentation, and terminating with smooth shells that 

 ränge from the Jura to the present time. The sutures become more sinuous and com- 

 plex in one of the subdivisions than in all other Nautiloids. The increase in number 

 of lobes and saddles begins in the Trias with Glymenonautilus, and ends with Aturia 

 in the Tertiary. 



I. Orthoceratida Hyatt. 



Orthoceracones and cyrtoceracones with smooth or ornamented shells, and not as a 

 rule contracted in gerontic stage ; apertures open throughout life. Although often short, 

 none are hrevicones, strictly speahing. Section circular or elliptical, very rarely oval. 

 Siphuncle with slightly nummuloidal, fusiform or tubulär segments, and generally near 

 the centre. 



Family 1. Orthoceratidae M'Coy. 



Section circular or compressed, living chamher uncontracted or only slightly so, 

 and aperture always open. Surface smooth or with only transverse bands, rarely 

 longitudinal Striae, never longitudinal ridges. Siphuncle small (except in Baltoceras), 

 Segments fusiform or cylindrical, never nummuloidal. Deposits when present irregulär, 

 and gathered about the funnels as in the Gyrtochoanites ; no definite endosiphuncles ever 

 formed. 



Baltoceras Holm. Siphuncle large, but with short, straight funnels, and sheatlis 

 as in Orthoceras. Ordovician. 



Orthoceras Breyn (Figs. 1109, 1110). Long tapering orthoceracones and cyrto- 

 ceracones, smooth, or with only transverse Striae and growth bands. Siphuncle 

 generally larger than in Geisonoceras, centren or slightly dorsad of centre. Deposits 

 when present gathered about the funnels as in the Annulosiphonata. Silurian to Trias. 



Geisonoceras Hyatt (Fig. 1111). Similar to the last, but sides spreading more 

 rapidly, and siphuncle empty, centren or slightly ventrad of centre. Ordovician to 

 Carboniferous. 



