PAPERS OX GEOGRAPHY AND GEOLOGY 355 



The minute fauna that is found in limestones and shales 

 of different ages are varied. The unique character of 

 some of these forms led earlier workers to consider them 

 diagnostic of specific beds. As the evidence accumulates, 

 however, it becomes apparent that only in rare instances 

 is a single form diagnostic of a certain bed, and that 

 rather an association of forms is the better diagnostic 

 evidence. The search for horizon markers has been ex- 

 tensive and even skeletal parts have been made use of, 

 not as wholly diagnostic in themselves, but as corrobo- 

 rative evidence. 



Diminutive species of larger forms, such as brachio- 

 pods and pelecypods, are not uncommonly met with in 

 the microscopic study of sediments. The forms which 

 are more novel, however, and are receiving more atten- 

 tion at present, may be mentioned briefly. 



Ostracods: — small crustaceans with a bivalved shell. 



Foraminifera: — small protozoans with a skeleton of 

 calcium carbonate. Tlie tests are of varied design, some 

 being coin-shaped, some like a grain of wheat, and others 

 composed of a series of globular chambers arranged in 

 regular or irregular patterns. 



Bryozoa: — colonial or encrusting molluscoidea, some 

 of which possess skeletons with intricately developed 

 lace-like patterns. 



Diatoms: — very small one-celled plants, with siliceous 

 cell walls. These forms are not known in any of the Illi- 

 nois rocks. 



Other less common remains than those mentioned 

 above, but forms which in combination with others are in 

 cases of diagnostic significance, are sponge spicules, 

 Echinois spines, Productns spines, worm cast-, anelid 

 jaws, and fish teeth. 



The geological usefulness of this sort of work is ap- 

 preciated readily when it is considered that in many 

 places the bed rock is concealed beneath a covering 

 of later sediments, wind-blown material, or glacial drift, 

 and it is therefore impossible to determine its nature. 

 The bed rock may come to the surface at some distant 

 place and an examination of the outcrops be possible 

 there, but in some cases the beds at the outcrop are so 



