30 



enough to be visible. How large you will judge, since the 

 shell, which I hold between ray finger and thumb, the 

 Hyalea Tridentata is the original of the first of these 

 five diagrams ; these shells arc not often seen in collections. 



The fourth class is that of the Brachiopoda, or arm-footed, 

 so called from a pair of fringed arms by which they are sup- 

 posed to agitate the water and draw within their shells the 

 Infusoria and the particles which serve them for food. The 

 three former classes which have been described are univalves, 

 that is to say the shells are in one piece, the shell of the 

 snail for example ; but Brachiopoda are bi-valves, that is to 

 say they consist of two shells opening with a hinge. The 

 Brachiopoda, like the testaceous Cephalopoda, may be con- 

 sidered rather as the inhabitants of a former than of the 

 present world, for while there are 70 living species belong- 

 ing to this class, 1,000 are found fossil ; this, as it has been 

 already remarked, would not of itself be conclusive were it 

 not also that the extinct genera are twice as many as the 

 recent, and that some whole families have been extinct ever 

 since the Primary Geological Period. The Brachiopoda are 

 small, ranging from the size of a pea to that of a common 

 cockle, which some of them very much resemble ; but there 

 is one feature by which the Brachiopoda of this form may 

 be distinguished : The Brachiopoda live attached t» the 

 rocks by a fleshy pedicle, and there is a hole in the lower 

 shell through which this attachment passes ; this gives to 

 some of the shells very much the form of an antique lamp 

 with the hole for the wick, and hence the Brachiopoda have 

 been called lamp shells by the older naturalists. The i'orms 

 of these shells will be best understood by these diagrams, 

 on an enlarged scale, and here is a specimen of a ) iving 

 species of another form, the Lingula, with the pedicle at- 

 tached to it. 



The fifth class is that of the Conchifera ; this includes, 

 with the exception of the Brachiopoda, the whole immense 

 group of bi-valve molluscs, some of which are well known 

 otherwise than as mere objects of natural history, such as 



