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. Famrty 6. PorertoceraTipAr. 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. Faminy 7. Cyrtoceratipar. Shell slightly curved; the 
aperture simple ; the siphuncle wide and the septa approximated. Genus 
—Cyrtoceras, Goldfuss ; Devonian. 
Famity 8. Lirvrripar. Shell 
eoiled in one plane with the ter- 
minal part uncoiled ; the aperture 
contracted. Genera—JLitwites, Bar- 
rande ; Silurian. Ophidioceras, 
Barrande ; Silurian (Fig. 261, A). 
Famity 9. TROCHOCERATIDAE. 
Shell helicoidally coiled; dextral 
or sinistral ; the last whorl gener- 
ally uncoiled. Genera — Trocho- 
ceras, Barrande; Devonian. 
Adelphoceras, Barrande ; Devonian. 
Famity 10. Navutimipar. Shell 
coiled in one plane ; the aperture 
wide and simple; the siphuncle 
central. Genera—Nautilus, Lin- 
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. (Gtyroceras, de 
Koninck ; from the Silurian to f 
the Carboniferous, Hercocoras, 4, Nautilus mecronplalus creeping on a horizon 
Barrande ; Silurian. Ptenoceras, mic touteelss e eye a fe hod ae n, aan ; 
a . . pa, nuchal part 0 ne mantle; p.o.t, posterlor 
Hyatt ; Devonian (Fig. 261, B). ophthalmic tentacle; sh, shell. (After Willey.) 
Discites, MacCoy ; Carboniferous. 
Faminy 11. Bacrritrpar. 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. 
Fie. 293. 
Sus-OrpDER 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 
