No. 499] 



DWARF FAUNAS 



473 



1. A Change in the Normal Chemical Content of the 

 Water.— Any change in the environment of an animal 

 which is away from that best suited to its highest 

 development tends to its deterioration. If a species de- 

 velops best in normal sea water, then an increase or 

 decrease in the chemical content of the water should be 

 detrimental to the animal, and this detriment should be 

 expressed in the shell, since, as shown by Hyatt and 

 others, there is the most intimate relation betwen the soft 

 and the hard parts of an animal, the least injury in the 

 soft parts being immediately expressed in the growing 

 shell. This expression will usually take the form of a 

 dwarfing in size, thinning and smoothing of the shell or 

 development of bizarre form. 



The possible changes in the normal chemical content of 

 the water are exceedingly numerous and all doubtless 

 affect the animal to a greater or less degree. The follow- 

 ing appear to be some of the more important of such 

 changes which produce a dwarfing effect. 



(a) A Change due to Freshening of the Water.-Th&t 

 forms of animals find fresh water detrimental to 

 them appears to be indicated by the fact that at present 

 whole groups are excluded from it, as the Echinodermata, 

 Brachiopoda, Cephalopoda, Tunicata, etc. 



The manv streams emptying into the Black and Caspian 

 seas, make them fresher than the Atlantic Ocean. The 

 faunas of these are typically marine, but practically all 

 are dwarfed in size as compared with the same species 

 in the Atlantic. For example, the following Black Sea 

 species are considerably smaller in the Black Sea than in 

 the British seas: Littorina rudis, Cerithium adrersum, 

 Trochus umbilicus, Murex erinacnis, Xnssa n-tn-nh,t,i, 

 Cardium edule, Anomia ephippntm , Venus galhua . I > l!»»a 

 tenuis, Mactra triangula, Solen ensis, Pholas Candida, 

 etc. 1 



The common European cockles, Cardium, are large, 

 thick and rough shells, and thrive best under purely 

 marine conditions. The species found growing in brack- 



