No. 499] 



DWARF FAUNAS 



487 



a dwarf form of the species 0. anomioides Meek. Ostrea 

 lugubris Conrad is a small species resembling the large 

 0. blackii. 



A probable explanation of this fauna and its habitat 

 may be drawn from Metzger's observations 31 ' upon that 

 much-studied shell, Cardium edide. This upon the sand 

 shoals of the ocean flats is small, while it attains its maxi- 

 mum size off the rugged coasts of Norway and Scotland. 

 These Cretaceous strata with their sand, cross-bedding, 

 much fragmentary wood, and a marine fauna indicate a 

 shallow sea into which emptied some large river, a condi- 

 tion probably somewhat similar to that of the eastern 

 part of the North Sea at present. 



g. It often happens that while a portion of a certain 

 fauna is dwarfed, the rest are of normal size. Hyatt 40 

 discusses an excellent example of this mixture of dwarf 

 and normal forms from the Tertiary strata of Steinheim, 

 northwestern Germany. 



Into the Tertiary lake at Steinheim migrated four 

 varieties of Pkmorbis lev is. The descendants of these 

 are divided into the progressive series or those which 

 became larger and more robust, and the retrogressive or 

 those which became smaller, less robust and distorted. 



The dwarfing of the one series may be explained as a 

 sapping of the individual \s energy due to its being in an 

 unfavorable environment. It is common to note signs of 

 old age appearing immediately after a severe injury to 

 the animal. 42 



In gastropods this is often indicated by uncoiling and 

 rounding of the whorls, so that these may be looked upon 

 as signs of weakness in the animal. Hyatt discusses the 

 reason for this as follows: 



