ORGANIC DEPENDENCE AND DISEASE 
75 
characters of the capulicls 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, 1 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 2 
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 Wetherby. 
9. P. bifurcatus Wetherby. 
10. P. depressus Lyon and Cassiday. 
11. Pliysetocrinus ornatus Hall. 
12. P. ventricosus Hall. 
13. Eucladocrinus millebrachiatus Wachsmuth and 
Springer. 
1 ‘ 1 Geological Survey of Illinois/ ’ v. 5, 334. 1873. 
2 We are greatly indebted to Dr. Charles R. 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). 
