ITS CHARACTERISTIC ANIMALS. 
201 
come much less conspicuous, while Crabs and 
Lobsters are now the prominent representatives 
of the class. Among Mollusks, the Chambered 
Shells, hitherto so numerous, have become, as 
they now are, very few in comparison with the 
naked Cephalopods. The Nautili, however, re- 
semble those now living in the Pacific Ocean ; 
and some fragments of the Paper-Nautilus have 
been found, showing that this delicate shell was 
already in existence. There is one very peculiar 
type of this class, belonging to the Tertiaries, 
which should not be passed by unnoticed. It 
partakes of the character both of the Cretaceous 
Belemnites and of the living Cuttle-Fish, and is 
known as the Spirulirostra. Another very char- 
Spirulirostra. 
acteristic group among the Tertiary Shells is that 
of the Nummulites, formerly placed by natural- 
ists in immediate proximity with the Ammonites, 
on account of their internal partitions. This is 
now admitted to have been an error ; their posi 
tion is not yet fully determined, but they cer- 
tainly stand very low in the scale, and have no 
9 * 
