EXCAVATORS. 363 



all this there is nothing having the appearance of 

 accident. Where the Cliona is not, the shell is per- 

 fectly sound and untouched ; the terminal twigs are 

 all alike delicate, and of similar character, penetrating 

 the hard, perfect substance, the main stems become 

 gradually and proportionately thick, and the anas- 

 tomosis, though somewhat irregular, is identical 

 throughout. 



If the sponges are incapable of excavating the 

 chambers in which they conceal themselves, it is 

 inquired how we shall account for the dendritic 

 cavities. " They are evidently not the result of 

 decay, neither are they the burrows of worms ; which, 

 when in shell, or other hard calcareous substance, are 

 always linear, sometimes cylindrical, often depressed, 

 never lobed, and frequently double, that is, with two 

 channels, divided by an elevated ridge. And so 

 different are they in their general appearance, that it 

 is very easy to point out which is the excavation of 

 the worm, and which that of the Cliona, when the 

 burrows of the two interfere with each other, which 

 not unfrequently occurs." 



This writer then proceeds to point out that the 

 walls of the cavities, inhabited by the sponge, are dis- 

 tinctly punctured in a peculiar manner, which must 

 be caused by the character of the surface of the in- 

 habitant. So certain a test is this stated to be, that 

 by it alone the nature of the excavations in fossil 



