SUB-ORDER D SCHISTOCHOANITESOETHOCHOANITES 517 



Dwightfs remarkable genera ''"'" i'" 1 < t Mieb.M- c,r<>\\\> m-ar l'oiighki-1-p...ii-, NVw V-.rk. 



They an- ]>rol talily diiv.-tly <Miin-ctrd with //,/./*./////>. Tin- lypii-al form of tin- 

 sul '-order is, of course, Conoceras, I'ronn. 



Oyrtocerina, Bill. l'.iv\ -iconic cyrtoceraeones. Siphiinc]<- 

 large, on tin- connive side and empty, luit having inti-nial ridg<-> 

 nli'-rnatiiig with septa of tin- camera.-. These ridges appear to 

 indicate allinily with ('minn'm*. Ordovician. 



Conoceras, Bronn (Bathmocercu, Ban. . !'!-. 1059). Brevi- 

 conic ortnoceracones, known only in their later Mage.- of dev-l<j- 

 meiit. Sijiliiincle <if niddci-atc >i/.c, suli-\-fiitran. Kunin-ls 

 reaching half-way across -a<-h cann-ra, >tiM-j.ly inclin.-l orad, 

 and split on the outer side. Closure of the walls effected l.y 

 a plate extending IVom tin- apical opening of each fmmrl 

 through the I'linnd iiself orad to the apical opening of the n.-\t 

 beyond, and projecting into the interior as a flattened fold, F[( . 



which is incomplete or open along the central axis. Thoe conoceras pro 

 internal collars or flat semiconical rings have leen described as Barr. (H.iovici.i 

 complete cones (Dwight). Ordovician. SinfK' "' 



Sub-Order D. ORTHOCHOANITES. Hyatt. 



Gerontic stages have uncontracted volutions and open apertures, except in a few 

 uncoiled phylogerontic genera. Siphuncular segments may be slightly nummuloidal, 

 fusiform or tubular, but are never markedly nummuloidal, nor are the funnels bent 

 sharply outwards as in Cyrtochoanites. Deposits formed only in the siphuncles of Ortho- 

 ceratidae and Kionoceras, and in them they are irregular and no endosiphuncles occur ; 

 other genera have empty siphuncles. Funnels, as a rule, both longer and straighter than 

 in Cyrtochoanites, 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 Palaeozoic shells, continuing in the Trias 

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

 range 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 numlu-r 

 of lobes and saddles begins in the Trias with Cbjmenonautilus, and ends with Af>!ri<.< 

 in the Tertiary. 



I. ORTHOCERATIDA. 



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 brevicones, strictly speaking. Section circular or elliptical, very rarely oval. 

 Siphuncle with slightly nUmmuloidal, fusiform, or tubular segments, and generally near 

 the centre. 



Family 1. Orthoceratidae. 



Section circular or compressed, living chamber ungontracted or only slightly so, 

 inxl 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 irregular, 

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

 formed. 



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

 as in Orthoceras. Ordovician. 



