36 IOWA STUDIES IN NATURAL HISTORY 



The greater number of erinoid families indicates that the 

 recent erinoid fauna, if not actually older than the recent ophi- 

 uran fauna, at least developed earlier, as the various forms have 

 become more definitely circumscribed and segregated into well 

 differentiated units. The relative state of undevelopment of the 

 ophiuran fauna is also indicated by the much larger number of 

 genera in each family and of species in each genus, as well as by 

 the larger number of species in each genus in any given locality. 



Only about one-half of the higher groups of crinoids are repre- 

 sented in the West Indies, while all of the higher groups of 

 ophiurans occur there. One erinoid family, the Holopodidae, is 

 known only from the Caribbean Sea. Assuming that the Ma- 

 layan region is the present center of distribution for marine 

 animals and the place of origin of most recent types, this is 

 easily accounted for by the much greater facilities for dispersal 

 possessed by the vast majority of the ophiurans, through their 

 pelagic young. 



Only 21% of the known erinoid genera are found in the West 

 Indies, while 46%, or more than twice as many, of the ophiuran 

 genera occur there. This also is probably due to the greater 

 facilities for dispersal possessed by the ophiurans. 



Of the known ophiuran species 18% occur in the West Indies, 

 as against 9%, half as many, of the known crinoids. This would 

 seem to indicate that as the Caribbean erinoid fauna had grown 

 old and become crystallized along definite lines no new acces- 

 sions had come in from the rapidly developing fauna of the 

 Malayan region, whereas the interchange of ophiurans from east 

 to west has kept the ophiuran fauna younger. 



Among the crinoids 53% of the Caribbean genera are not 

 represented elsewhere, while among the ophiurans the number is 

 less than half as large, only 24%. This is the result of the 

 crystallization of the West Indian erinoid fauna and the lack of 

 interchange with other faunas, which has taken place to a much 

 greater degree than in the case of the ophiurans. 



Exactly the same explanation holds for the fact that 37% of 

 the endemic genera of crinoids are monotypic, while only about 

 half as many of the ophiuran genera, 20%, are monotypic. 



The 192 genera of ophiurans may be classified according to 

 their geographical ranges as follows: 



