CLASS V 



CEPHALOPODA 



607 



Family 23. Estonioceratidae Hyatt. 



Gyroceracones and discoidal nautüicones having sUghtly depressed, hroad, rapidly 

 increasing hiangular sections in the young, tut hecoming depressed oval or depressed 

 suh-trigonal in later stages. Siphuncle variable in position. 



Estonioceras Noetl. Ordovician. Edaphoceras, Remeleoceras, Lophoceras Hyatt ; 

 Diodoceras Hyatt. Type D. (Endolohus) avonense (Dawson). Carbon iferous. Digonio- 

 ceras Hyatt. Jura. 



Family 24. Nautilidae Owen. 



Nautilicones wiih more or less involved volutions, the siphuncle slightly nummuloidal 

 and variable in position, but never near either the dorsiim or venter except in the young, 

 when it is frequently either near the dorsum or is centren. Biangular stage much 

 abbreviated or absent, the trigonal stage present in 

 most Shells for a more or less prolonged period, but 

 developing invariably by spreading of the venter into 



FifJ. 1122. 



Nautilus pompilius Linn. 

 Recent. Portion ofconch showing 

 linear cicatrix at apical end (after 

 Hyatt). 



Fio. 1123. 



Apical Chamber and first 

 volution of N. pompilius, 

 sectioned longitudinally. 

 s, Siphuncle; c, Blind 

 origin of caecum; x, Euipty 

 Space or uuibilical Perfora- 

 tion (after Branco). 



Fi«. 1124. 



Nautilus intermedius Sowb. Middle 

 Lias : Hinterweiler, Würtemberg. 



tetragonal, reniform or hemispherical outlines. Never decidedly dis- 

 coidal, although the umbilicus 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 the lohorls are in 

 contact. Annular lobes often developed at an early stage, but liable 

 to disappear in the adult ; absent in some Tertiary species. 



Genoceras Hyatt. Jura. Gymatoceras Hyatt. Cretaceous. 

 Eutrephoceras Hyatt. Cretaceous and Tertiary. 



Nautilus Linn. (Figs. 1122-1125). The young resembling 

 adults of Digonioceras until a late stage, and adults of primitive 

 species (like N. umbilicatus) similar to Genoceras. Sutures slightly 

 inflected, with faint ventral lobes ; annular lobes present. Volu- 

 tions sub-globose, and umbilical Perforation comparatively large. 

 Siphuncle centren in the apical camera, but later becoming ventrad 

 of centre, Tertiary (?) to Recent. 



Fio. 1125. 



NmUilns gcinitzi 

 Pictet. Tithonian ; 

 Stramberg, Moravia. 



