ORGANIC DEPENDENCE AND DISEASE 75 



characters of the capnlids because of the shell deformations 

 resulting from these attachments. So far as we have suc- 

 ceeded in listing these concurrences they are shown in 

 eighteen species of crinoids. Doubtless others have been 

 recorded which we may have overlooked. It was a matter 

 of comment by Meek and Worthen, who first took especial 

 notice of the frequency of these combinations/ that certain 

 crinoids seemed always to carry certain snails, intimating 

 thus a definite affinity of one for the other ; and to anyone 

 confronted by an array of these combinations this purpose- 

 ful predilection is very impressive. 



List of Recorded Carboniferous Crinoids 

 WITH Parasitic Gastropods^ 



1. Platycrinus hemisphericus Meek and Worthen. 



2. P. pileiformis Hall. 



3. Strotocrinus regalis Hall. 



4. Gilbertsocrinus tuberosus Lyon and Cassiday. 



5. G. typus Hall. 



6. Dorycrinus immaturus Wachsmuth and Springer. 



7. Agaricocrinus americanus Roemer. 



8. Pterotocrinus acutus Wetherhy. 



9. P. bifurcatus Wetherhy. 



10. P. depressus Lyon and Cassiday. 



11. Physetocrinus ornatus Hall. 



12. P. ventricosus Hall. 



13. Eucladocrinus millebrachiatus Wachsmuth and 

 Springer. 



1" Geological Survey of Illinois," v. 5, 334. 1873. 



2 We are greatly indebted to Dr. Charles E. Keyes for his investigation and 

 record of these associations as registered in literature and in the extensive 

 collection of crinoids at that time belonging to Dr. Charles Wachsmuth, now 

 by the generosity of Mr. Frank Springer the possession of the National Mu- 

 seum. Doctor Keyes 's papers on this subject of the species of Platyceras 

 which consort with the Mississippian crinoids were printed in 1888 ("Proc. 

 Amer. Phil. Soc," v. 25) and 1890 ("Proc. Acad. Nat. Sci.," Phila., v. 42). 



