72 
ORGANIC DEPENDENCE AND DISEASE 
growth and expansion of the shell after attachment was 
effected, is seen in the adaptation of the latter to the irregu- 
larities of the crinoid dome. The adjustment perfectly 
covers the anal region and is so effected as to avoid un- 
profitable surfaces. 
In all the crinoids taken from the extensive plantations 
uncovered at Vincent, N. Y., in the Hamilton shales (Mid- 
dle Devonian), the most striking assemblage of these fos- 
sils ever found in the Devonian rocks, not a single instance 
has been observed of this conjunction. We have, however, 
a unique occurrence from another locality, of one of the 
species represented in this colony, 
Rhodocrinus spinosus Hall, in which 
a very marked case of this parasitism 
is displayed, the gastropod covering 
the aboral orifice with the anterior 
part of its mouth, the rest of the 
aperture extending downward over 
the sides of the calyx. This is fre- 
quently a well-marked and perhaps 
normal position for the shell, and 
the necessity of keeping the month close to the source of 
food, forces the shell growth to be eccentric from this 
point and thus permits the shell margin to expand in any 
convenient direction. 
In the Upper Devonian we have record of attachment of 
a broad-mouthed capnlid to Melocrinus hieroglyphicus 
Goldfuss and a smaller species to M. globosus Fraipont. 
These are from the Frasnian rocks of Senzeille, Belgium, 
and have been figured by Fraipont . 1 They are sufficient 
proof of the widespread practice of this parasitism in the 
Devonian age. 
i“Ann. Soe. Geol. de Belgique,” Mem. X 1883, plates 2 and 5. Fraipont 
thinks the latter case mentioned in which the attached capnlid spreads over 
the side of the calyx, merely casual commensalism, but it is evident that the 
shell covers the position of the anal orifice. 
Fig. 59. Rhodocrinus 
calyx 'with heavy snail 
shell over anal aper- 
ture. Middle Devo- 
nian. 
