ORGANIC DEPENDENCE AND DISEASE 
67 
In tracing tlie history of the combination onr starting 
point is the rich marine fauna of the Cincinnati shale which 
lies near the summit of the Ordovician, tlms immediately 
antedating the advent of Silurian time. We have on a pre- 
Fig. 52. The snail Cyclonema 
enclosed within the arms of the 
crinoid Glyptocrinus in an at- 
titude of feeding. Ordovician 
age. 
Fig. 53. A cyst id, Caryocrinites 
carrying an attached snail 
which covers the apertures of 
the summit. Silurian age. 
vious occasion pointed out the free concurrence of the liolo- 
stomatons snail Cyclonema bilix, one of the abundant 
members of the fauna, with the crinoid Glyptocrinus deca - 
dactylus. In this case the shell is found to lie, inclosed by 
the arms of the crinoid, in a position that would give it full 
advantage of the waste from the crinoid, but without fixa- 
tion. It had been drawn to the crinoid by an obvious sup- 
ply of food but it was free to go and come as it was im- 
pelled. And yet the shell was of such build that its aperture 
was quite admirably adapted to fixation — a round thin edge 
without any irregularities which might lessen the effective- 
ness of the attachment. 
If now we may turn from the crinoids to the closely allied 
group of the Cystids in which the alimentary parts have 
essentially the same alignment, we find a case, long ago 
figured by Hall in the “Palaeontology of New York” (v. 2, 
pi. 49a, fig. 1 d, 1847), of an apparent attachment of a similar 
round-monthed shell (Stropliostylus) to the anal surface of 
the cystid Caryocrinites ornatus , of the Rochester shale 
