526 



MOLLUSCA 



SUB-KINGDOM VI 



VIII. DlGONIOCERATIDA. 



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. Estonioceratidae. 



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. nov. 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 in 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 



FIG. 1073. 



Nautilus pompilius, Linn. 

 Recent. Protoconch showing 

 linear cicatrix at apical end (after 

 Hyatt). 



volution of N. pompilius, 

 sectioned longitudinally. 

 s, Siphuncle; c, Blind 

 origin of caecum; x, Empty 

 space or umbilical perfora- 

 tion (after Branco). 



FIG. 1075. 



Nautilus intermedius, Sowb. Middle 

 Lias ; Hinterweiler, Wiirtemberg. 



is often open. More specialised forms have a minute umbilicus, 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 jhe 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. Eutrephoceras, 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. umbilicatus] similar to 



