MOLLUSCA — CEPHALOrODA. 
1G3 
Limestone of Ireland, lias its outer face so flattened that the Gallery- 
cone is almost triangular in section. Hcrcoceras, from the 
Devonian rocks of Bohemia, has a row of projecting tubercles 
arranged along its lengtli ; tlie shell is not very closely 
coiled, and its aperture is partly closed (Fig. 92 h). The 
Carboniferous shell A-phckceras is flattened, and so loosely 
coiled that there is usually an open space in the centre; 
ornament is often well marked on the inner whorls, but 
dies away, and gives place to transverse lines of growth in 
the later formed part of the shell (Fig. 92 c). The cone is 
sometimes grooved along its length, but such grooves are 
more pronounced in its contemporary, Vestinmdilus (Fig. 92 rf). 
Blunt tubercles are sometimes found on the sides of 
Tcmnocheilus, which occurs in Devonian, Carboniferous, and 
Triassic rocks, and has a flattened outer face with angidar 
sides. Plmronautilus, which lasts from Devonian to Triassic Table-cases 
times, is remarkable for its tubercles and transverse ribs, more 2 & 3 . 
like the ornament of an ammonite. 
The Carboniferous Asymptoceras has a smooth cone. Table-ease 
widening rapidly, and with the margin of the aperture drawn 2. 
out into a naiTOw projection on each side ; its septa are simple 
as in Nautihis, but the siphuncle is near the margin. In 
Subclymcma, on the other hand, the sutures are so folded as 
to resemble those of some goniatites. 
From the Triassic Limestones of the Austrian Alps are Table-case 
shown a number of shells, generally resembling that of the 3- 
recent Nmdilus, but separated by various minor differences. 
Even more like the modern Nautilus are the shells of Jurassic 
age. Among these is a specimen of Nautilus clausus from 
the Bajocian rocks, near Caen, showing impressions of the 
shell-muscles. A large Nautilus ornahis Irorn the Inferior Between 
Oolite of Sherborne, Dorset, shows the rapidly widening cone. Wall-cases 
the lines of growth, and longitudinal grooves. The Gault 
furnishes several nautili in which the shell is well preserved, 
and in a young shell of Nautilus scviiundatus, ivom the Upper 
Greensand of Warminster, the iridescence of the mother-o’- 
pearl septa may still be seen. This last species often attains a 
great size, as shown by specimens in tlie Wall-case. From 
the Upper Cretaceous Limestone of Lebanon comes Nautilus Wall-case 
lihanoticus, in two specimens of which the jaw-beaks may be 2. 
seen. The nautili of the Chalk are often large, but the 
internal casts are usually all that is found. 
Although it used to be said that the genus N'autilus 
persisted without change from early Palaeozoic times to the 
