SUB-ORDER B HOLOCHOANITES—MIXOCHOANITES 515 
and taper off at the centre into a spire that is sometimes tubular and hollow, or again 
flattened and elliptical, This is the endosiphwnele. 
Ordovician and Silurian. 
Narthecoceras, Hyatt. Long, cylindrical, staff-like 
orthoceracones. Siphuncle large and filled with organic 
deposits having a radiating fibrous structure like the 
guard of a Belemnite. Endocones and an endosiphuncle 
developed. Septa continuous around the siphunele. 
Ordovician. 
Nanno, Clarke. Similar to the preceding, but endo- 
siphuncle present only at the apical end. Siphuncle 
close to the shell, so that sutures appear to bend 
apically into a lobe passing around the siphunele. 
Trenton Limestone. 
Family 2. Piloceratidae. 
Shorter and stouter orthoceracones and cyrtoceracones 
with relatively larger siphuncles than in Endoceratidae, 
and more variable in their internal deposits. Septa are 
more concave and sutures more sinuous. Camerae empty 
and funnels similar, 

Fic. 1056. 
Piloceras, Salter. Breviconic cyrtoceracones with pi goceras proteiforme, Hall. Ordo- 
very large siphuncle and well-defined endocones. vician; New York. Longitudinal 
Peas 2 section showing funnels and endo- 
Ordovician. ; cones. 
Family 3. Cyrtendoceratidae. 
Gyroceracones and nautilicones having large siphuncles filled with organic deposits or 
empty, but with endocones obscure or absent, and no endosiphuncles. 
Cyrtendoceras, Remelé. Gyroceracones with siphuncle near the dorsum and filled 
with calcareous deposits. Ordovician. 
Sub-Order B. MIXOCHOANITES. Hyatt. 
Orthoceracones and cyrtoceracones having expanded living chambers with contracted 
apertures in the gerontic stage of specialised genera. The oldest septa are bent sharply 
orad, forming a series of dorsal saddles, and the siphunele becomes highly modified. 
Primitive genera have the septa deeply concave or approximately sub-conical, the siphuncle 
small and empty, and the septa sometimes more or less tmperfect on the ventral side in the 
gerontic stage. Specialised forms have siphuncles with short, straight funnels in the 
young, and in the ephebic stage collars are built around the oral openings, thus becoming 
parallel to some forms of Goniatitidae that have similar composite funnels. 
Family 1. Ascoceratidae. 
Cyrtoceracones, smooth or annulated. Siphuncle with long funnels only in the young 
and later stages of primitive genera, but collars are added in later stages of specialised 
forms, and segments become nummuloidal in gerontic stage. Septa often more or less 
imperfect around siphuncle and on the ventral side, 
Choanoceras, Lindstr. Sections depressed elliptical. Gerontic stages have no 
saddles, and living chamber uncontracted. Ordovician and Silurian. 
