2 PALEONTOLOGY 



that it is really a hollow shell (the test) composed of two 

 main portions in contact along their edges. These are 

 called the valves of the shell, and the valve which carries 

 the circular opening is called (for reasons to be seen 

 presently) either the ventral valve or the pedicle valve ; the 



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FIG. i. TEREBRATDLA INTERMEDIA (J. SOWERBY). 



a, Dorsal view ; b, ventral view ; c, punctation ; d, side view ; e, umbo, 

 foramen, and deltidial plates of ventral valve, a, b, d, Natural size ; 

 e, X 2 ; c , greatly enlarged. (Original. ) 



other is called the dorsal or brachial valve. The rounded 

 protuberance of the ventral valve is called its beak or 

 umbo. The dorsal valve has a similar, but much less 

 prominent umbo, without any opening. The end of the 

 shell at which lie these umbones is regarded as the hind 

 or posterior end, the opposite end being the front or 



