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- 
tobrachiatus Hall, which occurs in the same fauna, carries 
Fig. 57. Two specimens of a spiny crinoid, 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 the calyx of Megistocrinus with attached Platyceras. 
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 
