CEPHALOPODA. 195 
Shell nautiloid ; whorls separate ; siphuncle excentric, radiated. 
Fossil, 17 species. Upper Silurian—Trias? North America 
and Hurope. 
' THORACOCERAS, Fischer, 1844. 
Synonym, Melia, Fischer (not L.). 
Type, T. vestitum. 
Shell straight, elongated, conical, with a small lateral straight 
siphuncle. 
Fossil, 20 species. Lower Silurian—Carb. United States 
and Europe. 
NoTHOCERAS, Barrande, 1856. 
Shell nautiloid, slightly inyolute ; septa slightly arched, with- 
out lobes. 
Fossil, 1 species. Upper Silurian. 
Famity ITI.—AMMONITIDA. 
Shell. Body-chamber elongated ; aperture guarded by processes, 
and closed by an operculum; sutwres angulated, or lobed and 
foliated ; siphwncle external (dorsal, as regards the shell). 
The shell of the ammonitide has essentially the same structure 
as that of the nautilus. Itconsists of an external porcellanous* 
layer, formed by the collar of the mantle only and of an internal 
nacreous lining, deposited by the whole extent of its visceral 
surface. There is an ammonite in the British Museum, evidently 
broken and repaired during the life of the animal,+ which shows 
that the shell was.deposited from within. In some species of 
ammonites the collar of the mantle forms prominent spines on 
the shell, which are too deep for the visceral mantle to enter; 
they are therefore partitioned off (as in A. armatus, Lias) from 
the body whorl and air cells, and not exhibited in casts. 
The baculites and ammonites of the section cristati acquire, 
when adult, a process projecting from the outer margin of their 
shell. Certain other ammonites (the ornati, coronati, &c.) form 
two lateral processes before they cease to grow (Pl. III., Fig. 5). 
As these processes are often deyeloped in very small specimens, 
it has been supposed that they are formed repeatedly in the life 
of the animal (at each periodic rest) and are again removed when 
growth recommences. These small specimens, however, may 
be only dwarfs. In one ammonite, from the inferior oolite of 
Normandy, the ends of these lateral processes meet, ‘‘ forming 
* Its microscopic structure has not been satisfactorily examined; Professor Forbes 
detected a punctate structure in one species. 
t A. serpentinus, Schloth, U. Lias, Wellingboro. Rev, A. W. Griesbach. 
K 2 
