MOLLUSCA — CEPHALOPODA. 



159 



Fig. 89.— Three- 

 lobed aperture. 



away from it towards the centre, the space between the 

 two became filled with calcite. These structures are very 

 peculiar and often puzzling, especially when the inner tubes 

 are found apart from the outer shells ; in Huronia from 

 the Silurian rocks of North America they look like fossil 

 backbones, whence one species is named Huronia verte- 

 bralis. Some of the shells of Adinoceras must have been 

 immense, probably exceeding 10 feet (3 metres) in length, 

 for there is exhibited the body-chamber of Actinoceras 

 giganteum, which has a diameter of about 11 inches ; and a 

 huge fragment, the whole of which appears to be septate, 

 measures 2 feet 5 inches in length, the diameter of the 

 larger end being about 8 inches, that of the 

 smaller about 4J- inches. This genus, which 



possibly occurs in the Cambrian, ranges 

 through the Ordovician, Silurian, and 

 Devonian to the Carboniferous Epoch. 



Many of these Palaeozoic cephalopods 

 are peculiar in that, after the shell has 

 swollen out somewhat rapidly, perhaps to 

 accommodate the ripening eggs, it again 

 contracts in diameter, and not only this but 

 in some genera the edges of the shell close 

 over the opening of the body- chamber, 

 leaving only narrow apertures. Poterioceras 

 is one of the simpler types. The shell is 

 pear-shaped, the earlier part being narrow 

 and divided into chambers perforated by 

 a siphuncle which is somewhat inflated 

 between the septa ; the upper and larger 

 portion contained the animal. The whole 

 shell is usually slightly curved, and its 

 aperture not closed in. Fine examples of this genus come 

 from the Carboniferous Limestone of Ireland. It must have 

 attained considerable dimensions, for an example of Poterio- 

 ceras cordiforme from the red Carboniferous Sandstone of 

 Closeburn, Dumfriesshire, is 9 inches long and 7J- inches in 

 its greatest diameter. The shells of the Silurian Gomphocera- 

 tidse are more egg-shaped and the opening is narrowed to a 

 T-shape by the ingrowth of the margin (Fig. 89). The ends 

 of the crosspiece and upright of the T are widened, and we 

 may suppose that through the lower opening the funnel 

 could eject its stream of water while some arms could 

 emeroje throu<4h the 



Galle: 

 VII 



Fig. 90.— Four 

 lobed aperture. 



upper paired openings. Some allied 



