MOLLUSCA — CEPHALOPODA. 



163 



Limestone of Ireland, has its outer face so flattened that the 

 cone is almost triangular in section. Hercoccras, from the 

 Devonian rocks of Bohemia, has a row of projecting tubercles 

 arranged along its length ; the shell is not very closely 

 coiled, and its aperture is partly closed (Fig. 92 h). The 

 Carboniferous Apheleceras [JJiscites] 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, Vestinautilus (Fig. 92 rf). 

 Blunt tubercles are sometimes found on the sides of 

 Temnoclieilus, which occurs in Devonian, Carboniferous, and 

 Triassic rocks, and has a flattened outer face with angular 

 sides. PleuroncmtiliLs, which lasts from Devonian to Triassic 

 times, is remarkable for its tubercles and transverse ribs, more 

 like the ornament of an ammonite. 



The Carboniferous Asymptocc ras has a smooth cone, 

 widening rapidly, and with the margin of the aperture drawn 

 out into a narrow projection on each side ; its septa are simple 

 as in Nautilus, but the siphuncle is near the margin. In 

 Sitbclymenia, 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 

 shown a number of shells, generally resembling that of the 

 recent Nautilus, 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 orna.tus from the Inferior 

 Oolite of Sherborne, Dorset, shows the rapidly widening cone, 

 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 scmiundatus, from 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 the Wall-case. From 

 the Upper Cretaceous Limestone of Lebanon comes Nautilus 

 libanoticus, in two specimens of which the jaw-beaks may be 

 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 Nautilus 

 persisted without change from early Palaeozoic times to the 



M 2 



Gallery 

 VII. 



Table-eases 

 2&3. 



Table-ease 

 2. 



Table-ease 

 3. 



Between 

 Wall-cases; 

 13 & 14. 



Wall-ease 

 2. 



