j, CONCHIFEKA. 233 



Thus, caJceola, cJionetes, leptana, productus, are found only 

 in the palaeozoic rocks. Spirifer extends into the lias. 



Atrypa, gypidium^ and pentamerus, are ancient forms of 

 terebratula, which lived only in the primary epoch, whilst 

 the typical form of that genus nourished in the secondary 

 periods to such an extent that entire beds are sometimes 

 composed of its shells. A few species live in our modern 

 Beas. 



FIGS. 162, 163. Terebratula with the valves in contact, from the coralline 

 crag of Sudburn, Suffolk. 



The COKCHIFEEA are all enclosed in bivalved shells 

 articulated together in a hinge-like manner. The animal is 

 so placed in the shell that its back corresponds to the hinge, 

 and the one valve covers the right, and the other the left 

 side of the body; a position the reverse of that of the 

 brachiopods, where one valve is dorsal and the other 

 ventral. 



The shell is lined with a vascular membrane called the 

 mantle, which surrounds the body and secretes the shell. 

 The animal is attached to its calcareous envelope by one or 

 more muscles to close the valves and antagonise the elastic 



