No. 499] 



DWARF FAUNAS 



489 



and New Jersey coasts, in normal marine or but slightly 

 freshened water. An average-sized fossil specimen of 

 Trivia trivittata is three eighth inch long with a greatest 

 diameter of three sixteenth inch. An average size for 

 those off the Massachusetts coast is two third inch by 

 one third inch. The smaller specimen is less strongly 

 ribbed and less nodose. 



One of the larger of the fossil specimens of Mullnia 

 lateralis measures five sixteenth inch by one fourth inch. 

 The young shells off the coast are small, thin, with 

 margins subequally rounded and beaks inconspicuous and 

 nearly touching each other; this description applies to all 

 of the Hudson River specimens. It does not seem prob- 

 able, however, though possible, that so many shells could 

 be gathered at random as was done by the drill without 

 getting some adults. The more probable explanation 

 seems to be that these fossil individuals were living in an 

 unfavorable environment, a water less than normally 

 saline, and through a consequent sapping of vitality, were 

 not able to attain large size. 



In the first part of the above discussion was given what 

 appear to be some of the principal causes for the dwarfing 

 of invertebrate water-living faunas; these were illus- 

 trated by present-day examples. In the second part a 

 few fossil examples are described with a brief discussion 

 of the probable cause of the dwarfing in each case. 



The chief agency is apparently an abnormal habitat. 

 A species, for generations used to an environment of sea 

 water with a certain unvarying density, temperature, 

 clearness and depth, would become so accustomed to that 

 state of affairs that a change in one or more of the factors 

 would affect it unfavorably. An unfavorable environ- 



energy left for growth. There would thus result a 



Summary 



tinuance of life th; 

 the maintenance of 



