42 INVERTEBRATE AKIMALS. 



Group 124.— Genera GONIATITES (De Haan) ; and 

 CEEATITES (De Ha^n). Sutures of the shell sinuous, 

 but not ramified. Estimated number of species 175, 

 known only as fossils from the Devonian to the Trias. 



Family NAUTILID^. 



Group 125.— Genus ORTHOCERAS (Breyn) ; and allies. 

 op^oi, straight ; ^ipoLi, a horn. The Orthoceratites were 

 extremely abundant in the very ancient seas, the 

 Carboniferous Limestone in some parts being almost 

 made up of their remains. The shells sometimes attained 

 a length of six feet. Estimated number of species 200, 

 from the Silurian to the Trias. 



Group 126.— Genus NAUTILUS, vaurlxoc. Aristotle, 

 B.C. 312. Nautilus, the sole Hving representative of the 

 vast assemblage of tentacle-bearing Cephalopods, has, it 

 is said, been recognised in every geological formation 

 from the Silurian upwards. The series thus extended 

 includes more than 100 species ; 6 species are still 

 living, though the animal has rarely been observed, and 

 was first described by Professor Owen. The Nautilus 

 creeps, shell uppermost, on the bottom of the sea ; but 

 it occasionally ascends, and has been seen in shoals 

 floating on the surface after a storm. 



H Examples of Nautilus pompilius, N. umhilicatus, 

 N. macrom'phalus, and of a few fossil forms. 

 Upper Compartment. 

 Sections of the shell to shew the chambers and the 

 siphuncle. Specimen in liquid of the animal 

 within the shell. 



