OEGANIC DEPENDENCE AND DISEASE 



67 



In tracing the history of the combination our starting 

 point is the rich marine f anna of the Cincinnati shale which 

 lies near the summit of the Ordovician, thus immediately 

 antedating the advent of Silurian time. We have on a pre- 



Fig. 52. The snail Cyclonema 

 enclosed within the arms of the 

 erinoid GljT.it ocriuus in an at- 

 titude of feeding-. Ordovician 



Fig. 53. A cystid, Caryoerinites 

 carrjdng an attached snail 

 which covers the apertures of 

 the summit. Silurian age. 



vious occasion pointed out the free concurrence of the holo- 

 stomatous snail Cyclonema hilix, one of the abundant 

 members of the fauna, mth the erinoid Glyptocrinus deca- 

 dactylus. In this case the shell is found to lie, inclosed by 

 the arms of the erinoid, in a position that would give it full 

 advantage of the waste from the erinoid, but without fixa- 

 tion. It had been dra\\m to the erinoid 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. 4Qa, fig. Id, 1847), of an apparent attachment of a similar 

 round-mouthed shell (Stropho stylus) to the anal surface of 

 the cystid Caryoerinites ornatus, of the Rochester shale 



