BRACHIOPODA. 



I 99 



they are never composite, and by the possession of a bivalve, 

 calcareous, or sub-calcareous shell. They are commonly known 

 as " Lamp-shells," and are all inhabitants of the sea. All the 

 living forms are fixed to some solid object in their adult con- 

 dition ; but there is good reason to believe that many of the 

 fossil forms were unattached and free in their fully-grown con- 

 dition. From the presence of a bivalve shell, the Brachiopods 

 have often been placed near the true bivalve Molluscs (the 

 Lamellibranchiata) ; but their organisation is very much in- 

 ferior, and there are also sufficient differences in the shell to 

 justify their separation. 



The two valves of the shell in any Brachiopod are articulated 

 together by an apparatus of teeth and sockets, or are kept in 

 apposition by muscular action alone. As regards the contained 

 animal, the position of the valves is anterior and posterior, so 

 that they are properly termed the " ventral " and " dorsal " 

 valves. One of the valves is always slightly, sometimes greatly, 

 larger than the other, so that the shell is said to be " inequi- 

 valve" (fig. 139). On the other hand, a line drawn vertically 



Fig. -L^.R 



C, View of the base, a Ventral valve ; b Dorsal valve ; f Base ; c Beak ; k Foramen. 

 Lower Cretaceous. 



from the beak of the shell to its base (in fig. 139 B, from c to/), 

 would divide it into two equal halves, so that the shell is said 

 to be " equilateral." In the true bivalve Shell-fish (Lamelli- 

 branchiata), on the contrary, the valves of the shell are placed 

 upon the sides of the contained animal, so that they are " right " 

 and " left," instead of being dorsal and ventral. Further, the 

 two valves are usually of the same size (" equivalve "), and a 

 line drawn from the beak to the base would almost always 

 divide the shell into unequal halves ; so that the shell is " in- 

 equilateral." 



Ordinarily the ventral valve of the shell of the Brachiopods 

 is the largest of the two, and it is generally furnished with a 

 prominent curved "beak." Very commonly the beak is per- 



