SUB-ORDER D SCHISTOCHOANITES—ORTHOCHOANITES 517 
Dwight’s remarkable genera from the Quebee Group near Poughkeepsie, New York. 
They are probably directly connected with Holochoanites. The typical form of the 
sub-order is, of course, Conoceras, Bronn. 
Cyrtocerina, Bill.  Breviconic cyrtoceracones. Siphuncle 
large, on the concave side and empty, but having internal ridges 
alternating with septa of the camerae. These ridges appear to 
indicate affinity with Conoceras. Ordovician. 
Conoceras, Bronn (Bathmoceras, Barr.), (Fig. 1059). Brevi- 
conic orthoceracones, known only in their later stages of develop- 
ment. Siphuncle of moderate size, sub-ventran. Funnels 
reaching half-way across each camera, steeply inclined orad, 
and split on the 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 Gy) ocoras _praeposterum, 
internal collars or flat semiconical rings have been described as Barr. Ordovician (Htage 
. - D); Vosek, Bohemia (after 
complete cones (Dwight). Ordovician. Eee oe 

Fic. 1059. 
Sub-Order D. ORTHOCHOANITES. Hyatt. 
Gerontic stages have uncontracted volutions and open apertures, except in aw few 
uncoiled phylogerontic genera. Siphuncular segments may be slightly nummulordal, 
fusiform or tubular, but are never markedly nuwmmuloidal, 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 trreqular and no endosiphuncles oceur ; 
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 number 
of lobes and saddles begins in the Trias with Clymenonautilus, and ends with Aturia 
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 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 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. : 
