ORGANIC DEPENDENCE AND DISEASE 



71 



nevertheless there were lacking in this local fauna neither 

 crinoids nor snails of other species. The limpets are of the 

 usual type with small spires and broad whorls and not un- 

 like each other in general proportions. Hystricrinus punc- 

 tohrachiatus Hall, which occurs in the same fauna, carries 



Fig. 57. Two specimens of a spiny erinoid, Hystricrinus, with two different 

 species of snails. Devonian. 



no parasite so far as known and the later species of the 

 genus (H. ithacensis) is without such associates. The di- 

 rective impulse indicated in the adjustment of the limpets 

 to these crinoids is here undeniably clear. 



From the Middle Devonian limestone fauna of central 



Fig. 58. Two views of tlie calyx of Megistocrinus with attached Platyeei-as. 

 Middle Devonian. 



Iowa we have heretofore recorded the combination between 

 the crinoid Megistocrinus farnsworthi White and a large 

 Platyceras. This is known only in a single case and, as 

 shown by the figure here given, the evidence of gradual 



