12 PALAEONTOLOGY 



The protegulum of a brachiopod forms the apex of 

 each valve. Too often, owing to the exposed position of 

 the umbo, especially of the ventral valve, it may be 

 rubbed away in later life ; in some forms it is absorbed 

 in the enlargement of the pedicle-opening ; but in many 

 cases it persists through life. Growth consists in adding 

 to it both on its inner surface and on its margins to 

 the greatest extent at the anterior margins. During this 

 growth the shape of the shell may change, perhaps more 

 than once ; it may acquire surface-ornament, and this 

 may alter or disappear ; but (apart from wear and tear) 

 every stage is preserved in the adult shell, and if the 

 characters of an early stage are the same as those of the 

 adult shell of a brachiopod of earlier date, there is a 

 presumption that the latter may be ancestral to the one 

 under consideration. 



Growth by accretion is not possible, however, for all 

 parts of the skeleton : the loop, once formed, could not 

 grow so and remain a loop. As the valves increase in 

 size, repeated resorption and fresh secretion of the shelly 

 substance of the loop must take place. The difference 

 in development between the valves and the loop is like 

 the difference between that of a ram's horn and that of a 

 stag's antler. Hence the adult loop affords no evidence 

 of the stages through which it has passed. These can 

 only be determined by actual observation of the develop- 

 ment in living brachiopods, and by the fortunate preser- 

 vation of immature as well as mature specimens among 

 fossils. The former method has yielded results which 

 afford striking confirmation of the principle of palin- 



