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CHAPTER XVIII. 
MOLLUSCA CEPHALOPODA. 
‘‘Monstrum horrendum, informe, ingens.” — VIRGIL. 
Tue highest class of the Mollusca is the Cephalopoda, which has 
been divided by Professor Owen into two orders, Zetrabranchiata, or 
animals having four branchie, and the Dybranchiata, having two 
branchiz. The first order, Zéetrabranchiata, contains the family 
Ammonitide, with the fossil genera CGondatites and Ammonites, the 
family Orthoceritide, the fossil genera Gomphoceras and Orthoceras ; 
and the family Vautiiide, with the genus WVautilus. 
The name Cephalopoda, as already stated, is taken from the 
position of their feet, or more properly their arms, which are inserted 
in the anterior part of the head: in Greek xepdan, head, mods-rodds, 
Soot. 
The Cephalopodous Molluscs are indeed highly organised for 
Molluscs, for they possess in a high degree the sense of sight, 
hearing, and touch. They appear with the earlier animals which 
present themselves on the earth, and they are numerous even now, 
although they are far from playing the important part that was 
assigned to them in the early ages of organic life upon our planet. 
The Ammonites and Belemnites existed by thousands among the 
beings which peopled the seas during the Secondary epoch in the 
history of the globe. 
TETRABRANCHIATA. 
In this order the animal is creeping, protected with an external 
shell; the head is retractile within the mantle; the eyes are peduncu- 
lated ; the mandibles are calcareous ; there are very numerous arms. 
The shell is external, camerated, and provided with a siphuncle, or 
-membranous tube (Vauti/us), with a thin nacreous investment. In 
many fossil forms it consists of a number of funnel-shaped or bead- 
like tubes. This order differs from that of the Dibranchiata chiefly 
in their more numerous arms, which are quite tentaculiferous, in 
