INVERTEBRATE ANIMALS. 43 



nearly straight or sharply bent near the extremity ; also 

 genus Turrilites, in which the chambered tube forms a 

 beautifully spiral shell. Estimated number of species, 

 140, all known as fossils of the Cretaceous system. 



Group 123.— Genus AMMONITES (Bruguiere). Sutures 

 of the shell more or less ramified. Estimated number 

 of species, 530, occurring from the Trias to the Chalk. 

 II Specimens of Ammonites from the Wealden, Oolite, 

 Lias, Chalk, and Green-sand formations. 



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

 CERATITES (De Haan). 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. 

 6pQo§, straight; xspag, 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. vuvtIxo?-. Aristotle, 

 B.C. 312. Nautilus, the sole living 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 



