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MANUAL OF ZOOLOGY. 



Pteropods, are of comparatively gigantic size. Both commence 

 their existence in the Silurian or Upper Cambrian, and the 

 former is entirely Palaeozoic. The genus Conularia, however, 

 extends into the Mesozoic period, and is found in the Liassic 

 rocks. The Silurian fossils which form the genus Tentaculites , 

 though often referred to the Tubicolar Annelides, appear to 

 belong without doubt to the Pteropoda. The recent genera 

 Hyalea, Cleodora, and Cuvieria are represented in the Tertiary 

 period. 



Cephalopoda. The Cephalopods are largely represented in 

 all the primary groups of stratified rocks from the Lower Silu- 

 rian up to the present day. Of the two orders of Cephalopoda, 



Fig. 151. Shells of Secondary Cephalopods. i. Ancyloceras Matheronianus ; 2. Sca- 

 phites cequalis : 3. Crioceras Duvalii : 4. Hamites attenuatus ; 5. Turrilites cate- 

 natus t 



the Tetrabranchiata is the oldest, attaining its maximum in the 

 Palaeozoic period, decreasing in the Mesozoic and Kainozoic 

 epochs, and being represented at the present day by the single 

 form Nautilus pompilius. Of the sections of this order, the 

 Nautilida proper and the Orthoceratida are pre-eminently 

 Palaeozoic, and the Ammonitida are not only pre-eminently 

 but are almost exclusively Secondary. Of the abundance of 

 the two former families in the Silurian seas some idea may be 

 obtained when it is mentioned that over a thousand species 

 have been described by M. Barrande from the Silurian basin 

 of Bohemia alone. The Nautilida proper have gradually de- 

 creased in numbers from the Palaeozoic through the Secondary 



