846 



DIVISIONS OF THE CEPHALOPODA. 



Fig. 760. — Nautilus {Vestinautilus) Koninckii. 

 Carboniferous. 



groups, to some of which a generic value is now usually attached. 

 Using the name in the wide sense in which it has been commonly 

 employed, the genus may be said to range from the Ordovician to 



the present day ; but the 

 older types are almost cer- 

 tainly not congeneric with 

 those of later periods. 



The name of Discites is 

 given to forms generally in- 

 cluded in the comprehensive 

 genus Nautilus, in which 

 the shell is widely umbili- 

 cated, with a persistent cen- 

 tral vacuity ; the volutions 

 being four -sided, and the 

 septa showing a deep ventral lobe. The forms of this type are 

 Carboniferous. Temnocheilus (Devonian to Trias) and Tremato- 

 discus (Carboniferous) likewise include widely umbilicated Nautili, 

 in which the centre is perforated. A similar condition of parts 

 exists in Vestinautilus (fig. 760), but in this 

 type the whorls are trapezoidal in section and 

 are very wide towards the aperture. The 

 outer side of the shell is strongly keeled with 

 lateral longitudinal ribs separated by a dorsal 

 furrow. The genus is confined to the Carbon- 

 iferous rocks. 



Lastly, in Aturia the shell is involute and 

 destitute of an umbilicus ; and the sutures are 

 strongly bent in a zigzag manner, with a deep 

 lateral "lobe." The siphuncle is marginal and 

 is placed on the internal side of the shell, the 

 septal "necks" (fig. 743, .si) being very long 

 and being invaginated in such a manner as 

 to form a complete siphuncular investment. 

 The known species of Aturia are found in the 

 Eocene and Miocene deposits. 



Family ii. Bactritid^e. — This family in- 

 cludes only the genus Bactrites (fig. 761), in 

 which the shell is straight and conical, with a 

 circular or elliptical section, resembling that of 

 Orthoceras in form. The siphuncle is delicate and marginal in 

 position ; and the septal necks are long and funnel-shaped. The 

 sutures are undulated, with a backwardly-directed sinus correspond- 

 ing with the siphuncle. Owing to the characters of the suture- 

 lines, the genus Bactrites has been regarded as related to the 



K- 



Fig. 761. — Bactrites 

 elegans, Devonian, Ger- 

 many, of the natural 

 size. (After Sandberger 

 — copied from Zittel.) 



