CEPHALOPODA. 195 



Shell nautiloid ; whorls separate ; siphuncle excentric, radiated. 

 Fossil, 17 species. Upper SiluriarL — Trias ? North America 

 and Europe. 



Thoracoceras, Pischer, 1844. 



Synonym, Melia, Pischer (not L.). 



Type, T. yestitum. 



Shell straight, elongated, conical, witii a small lateral straight 

 siphuncle. 



Fossil, 20 species. Lower Silurian — Garb. United States 

 and Europe. 



NoTHOCERAS, Barrande, 1856. 

 Shell nautiloid, slightly involute ; septa slightly arched, with- 

 out lobes. 

 Fossil, 1 species. Upper Silurian. 



Eajmeilt III. — Ammonitid^, 



Shell. Body -chamber elongated ; aperture guarded by processes, 

 and closed by an operculum ; sutures angulated, or lobed and 

 foliated; siphuncle external (dorsal, as regards the shell). 



The shell of the ammonitidce has essentially the same structure 

 as that of the nautilus. It consists 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, f 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 thereiore 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 (PI. III., Pig. 5). 

 As these processes are often developed 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 Forbea 

 detected a punctate structure in one species. 



t A. serpentinus, Schloth, U. Lias, Wellingboro. Eev. A. W. Griesbach. 



k2 



