74 
ORGANIC DEPENDENCE AND DISEASE 
mac, Rhodocrinus and Melocrinus sp., where dispropor- 
tionate growth of the gastropod evinces lack of advan- 
tageous gravitational balance such as is seldom to be found 
in the following period when the parasitism had become 
set. We observe a greater variety in the form of the snail 
than appears later, and it is evident that some of these 
were better adapted than others to this mode of life. And 
we also see, from the instance of conjunction at a very early 
age in both parties to the combination, that the habit must 
be a long-time inheritance out of the past. 
We may now consider the expression of this parasitic 
adjustment as it appears in great frequency in the marine 
faunas of the next or Carboniferous age, especially in the 
calcareous deposits which compose the earlier or, as called 
in this country, the Mississippian stage of this series. 
62 63 
Fig. 62. Part of the dome of the crinoid Strotocrinus showing successive growth 
scars made by an attached Platyceras always keeping its anterior extremity 
over the anal aperture of the crinoid. Mississippian. (After C. R. Keyes.) 
Fig. 63. The crinoid Platycrinus hemisphaericus with the snail Platyceras infun- 
dibulum attached. Mississippian. (After C. R. Keyes. ) 
Such concurrences are noteworthy for the evidence they 
bring of the general dissemination of this habit which now 
attains its climax. So general is it that paleontologists 
have recognized the difficulty of determining the species 
