ORGANIC DEPENDENCE AND DISEASE 



85 



erto in the Cambrian, occasional in the Ordovician, more 

 common in the Silurian, they became abundant and wide- 

 spread in the Devonian and thence onward to the present 

 fauna and their activities now are known not to be confined 

 to the salt waters only/ 



Sponges. Excavations which seem referable to the activ- 

 ities of perforating sponges of types comparable to the liv- 

 ing Clionaand Thoosa, which 

 are familiarly known to bore 

 in calcareous remains, have 

 been observed by us in va- 

 rious brachiopod and acepha- 

 lous shells of the Devonian. 

 Our record of them does not 

 extend into the earlier fau^ 

 nas. We are holding the same 

 reservation regarding the 

 precise nature of some of 

 the borings here assigned to 

 sponges of this type because 

 of want of full comparative 

 evidence, but the assignment 

 is based on comparisons of 

 size and mode of growth as 

 between the microscopic tu- 

 bules of the algae and the large regular tubes of the worms. ^ 

 In a previous discussion of such perforating organisms^ we 

 instituted the generic designation Clionolithes for a group 

 which was based on the form described by McCoy* from 



1 See T. S. Collins. ' ' Some Perforating and Other Algae on Fresh Water 

 Shells"; ErytJiea, v. 5, p. 95. 1897. 



2 Comparison with the tnbules of living perforating sponges may be made 

 by reference to the work of Emile Topsent in the Archives de Zoologie Experi- 

 mentale, 2d ser., v. 5 bis, 1887, 1891; 4th ser., v. 7, 1907. 



3 ' ' Dependent Life. ' ' 



4 "British Paleozoic Fossils," 1855, p. 260, pi. 13, fig. 1, la. 



Fig. 69. Boring algae in the test of 

 the trilobite Odontoceplmlus selenu- 

 nis (Devonian), x 25. 



