596 MOLLUSCA phylum vi 



annulated orthoceracones. Funnels reach from septum of origination to the next 

 apicad of this, but no farther. Septa pass entirely around the siphuncle. Organic 

 deposits in the form of endocones, and taper off at tlie centre into a spire that is 

 sometimes tubular and hollow, or again flattened and elliptical. This is the enclo- 

 siphunde. 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 siphuncle. Ordovician. 



Nanno Clarke. Similar to the preceding, but endosiphuncle present only at the 

 apical end. Siphuncle close to the shell, so that sutures appear to bend apically into 

 a lobe passing around the siphuncle. Trenton Limestone. 



Family 2. Piloceratidae Hyatt. 



Shorter and stouter orthoceracones and cyrtoceracones loith 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. 



Piloceras Salter. Breviconic cyrtoceracones with very large siphuncle and well- 

 defined endocones. Ordovician. 



Family 3. Cyrtendoceratidae Hyatt. 



Gyroceracones and nautilicones having large siphuncles filled with organic deposits or 

 empty, hut with endocones obscure or absent, and no endosiphimcles. 



Gyrtendoceras Remele. Gyroceracones with siphuncle near the dorsum and filled 

 with calcareous deposits. Ordovician. 



Suborder B. MIXOCHOANITES Hyatt. 



Orthoceracones and cyrtoceracones having expanded living chambers with contracted 

 apertures in the gerontic stage of specialised genera. Tlie oldest septa arc bent sharply 

 orad, forming a series of dorsal saddles, and the siphuncle 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 imperfect on the ventral side in the 

 gerontic stage. Specialised forms have siphunchs with short, straight funnels in ths 

 younq, and in the ephcbic stage collars are built around the oral openings, thus becoming 

 parallel to some forms of Goniatitidae that have similar composite funnels. 



Family 1. Ascoceratidae Barrande. 



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 the gerontic stage. Septa often more or less 

 imperfect around the siphuncle and on the ventral side. 



Ghoanoceras Lindstr. Sections depressed elliptical. Gerontic stages have no 

 saddles, and living chamber uncontracted. Ordovician and Silurian. 



Aphragmites Barr. Only gerontic li\'ing chambers known ; these are similar to 

 those of Ascoceras, but have no internal sigmoidal dorsal saddles. Silurian. 



Ascoceras Barr. (Figs. 1 106, 1107). (Terontic living chambers internally contracted 

 by the formation of large sigmoidal saddles, and septa more or less incomplete 



