526 MOLLUSCA SUB-KINGDOM VI 
VIII. DiGonrtocERAtTIDA. 
Primitive forms constantly retain depressed volutions having a more or less biangular 
or sub-trigonal section ; specialised shells repeat these stages in the young, but subsequently 
become more involute, and the sections change to reniform, sub-quadrangular, or sub- 
elliptical. Shells smooth except in the single genus Cymatoceras. Aperture simple and 
open at all stages; gerontic living chamber only slightly contracted. 
Family 23. Hstonioceratidae. 
Gyroceracones and discoidal nautilicones having slightly depressed, broad, rapidly 
increasing biangular sections in the young, but becoming depressed oval or depressed 
sub-trigonal in later stages. .Siphuncle variable in position. 
Estonioceras, Noetl. Ordovician. Edaphoceras, Remeleoceras, Lophoceras, Hyatt ; 
Diodoceras, gen. noy. Type D. (End.) avonensis, Dawson sp. Carboniferous. Digonio- 
ceras, Hyatt. Jura. 
Family 24. Nautilidae. 
Nautilicones with more or less involved volutions, the siphuncle slightly nummuloidal 
and variable in position, but never near either dorsum or venter except im the young, 
when it is frequently either near the dorsum or centren. Biangular stage much 
abbreviated or absent, the trigonal stage present in most shells for a more or less pro- 
longed period, but developing invariably by spreading of 
the venter into tetragonal, reniform, or hemispherical 
outlines. Never decidedly discoidal, although umbilicus 

Fic. 1074. 
Apical chamber and first 

7 = volution of N. pompilius, 
Fia. 1073. sectioned longitudinally. 
Nautilus pompilius, Linn. s, Siphuncle; ec, Blind Fic. 1075 
Recent. Protoconch showing origin of caecum; x, Empty eb et 
linear cicatrix at apical end (after space or umbilical perfora- Nautilus intermedius, Sowb. Middle 
Hyatt). tion (after Branco). Lias ; Hinterweiler, Wirtemberg. 
is often open. More specialised forms have a minute wmbilicus, and in some cases it is 
completely hidden during the ephebic stage, although invariably open in the young. 
Zone of impression present on the dorsum before the whorls are in contact. Annular 
lobes often developed at an early stage, but liable to disappear in the adult ; absent in ~ 
some Tertiary species, 
Cenoceras, Hyatt. Jura, Cymatoceras, Hyatt. Cretaceous. Hutrephoceras, Hyatt. 
Cretaceous and Tertiary. 
Nautilus, Linn. (Figs. 1073-1076). The young resembling adults of Digonio- 
ceras until a late stage, and adults of primitive species (like N. wmbilicatus) similar to 
