THE CEPHALOPODA 



333 



ASCOCERATIDAE. Shell straight, ampulliform ; the summit truncated ; 

 the terminal chamber occupies nearly the whole length of the shell on 

 the ventral side. Genera Ascoceras, Barrande ; Silurian. Glossoceras, 

 Barrande ; Silurian. FAMILY 6. POTERIOCERATIDAE. Shell straight or 

 curved, fusiform, contracted at the two extremities ; the aperture simple ; 

 the siphuncle contracted at the levels of the septa. Genera Poterioceras, 

 MacCoy ; from the Silurian to the Carboniferous. Streptoceras, Billings ; 

 Silurian. FAMILY 7. CYRTOCERATIDAE. Shell slightly curved ; the 

 aperture simple ; the siphuncle wide and the septa approximated. Genus 

 Cyrtoceras, Goldfuss ; Devonian. 

 FAMILY 8. LITUITIDAE. Shell 

 coiled in one plane with the ter- 

 minal part uncoiled ; the aperture 

 contracted. Genera Lituites, Bar- 

 rande ; Silurian. Ophidioceras, 

 Barrande ; Silurian (Fig. 261, A). 

 FAMILY 9. TROCHOCERATIDAE. 

 Shell helicoidally coiled; dextral 

 or sinistral ; the last whorl gener- 

 ally uncoiled. Genera Trocho- 

 ceras, Barrande; Devonian. 

 Adelphoceras, Barrande ; Devonian. 

 FAMILY 10. NAUTILIDAE. Shell 

 coiled in one plane ; the aperture 

 wide and simple ; the siphuncle 

 central. Genera Nautilus, Lin- 



TUX, 



FIG. 293. 



naeus (Figs. 270 and 293) ; four 

 living species are known from the 

 Indian and Pacific Oceans : they 

 are gregarious, nocturnal animals, 

 living at some depth. Trocholites, 

 Conrad ; Silurian. Gyroceras, de 

 Koninck ; from the Silurian to 



thp ParhrniifpT-mia HW/VMMVIA Nautilus macromphalus creeping on a horizon- 



OaroomteroUS. Jlercoceras, tal surface, anterior view. a.o.t, anterior ophthal- 



Barrande ; Silurian. Ptenoceras. mic tentacle ; e, eye ; ho, hood ; in, infuudibulum ; 



TT T^ . /T ,. -., -r^v pa, nuchal part of the mantle; p.o.t, posterior 



Hyatt; Devonian (Fig. 261, B). ophthalmic tentacle ; sh, shell. (After Willey.) 



Discites, MacCoy ; Carboniferous. 



FAMILY 11. BACTRITIDAE. Shell straight, conical ; the siphuncle narrow 

 and marginal, and the siphuncular necks long and infundibuliform ; 

 septa united to the shell by an undulating line. Genus Bactrites, 

 Sandberger ; Silurian and Devonian. 



SUB-ORDER 2. AMMONITOIDEA. 



The Ammonitoidea are distinguished from the Nautiloidea by their 

 initial chamber, which is spheroidal like that of the Belemnitidae and 

 Spirulidae ; by their siphuncle, which is narrow and simple, whereas that 

 of the Nautiloidea is wide, but often reduced in diameter by internal 

 deposits ; by their septa, which are generally convex on the side nearer 



