BEACHIOPODA. 



231 



the Nautilus. This multilocular dwelling is rolled into a 

 variety of forms, being circular, scaphite, or turreted, in the 

 different genera. 



As it is most important that the student should possess 

 clear ideas of the classes of the Mollusca, we have exhibited 

 their most important characters in the subjoined table : 



'The body in the form of a sac, open"| 

 before, from whence the head I p 

 protrudes, surrounded by eight or f 

 more feet, or tentacula 



TUNICATA. In consequence of the absence of calcified 

 parts in the tunicata, their remains have not been recognised 

 in a fossil state. As the other classes of mollusca are nearly 

 all enclosed in shells, these external skeletons are well 

 preserved in the rocks of all periods, and form the most 

 numerous and valuable groups of fossil remains. 



The BKACHIOPODA, or arm-footed molluscs, are marine 

 animals enclosed in bivalved shells, attached to foreign 



