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. 



Fig. 62. Part of the dome of the crinoid Strotoerinus 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 Plati/cnnus liemispliaericus with the snail Platyceras infun- 

 f7i&M?M)tt 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 



