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 

 Fig. 59. Rhodo^us part of its mouth, the rest of the 

 calyx with heavy snail aperture extending downward over 

 shell over anal aper- the sidcs of the calyx. Tliis is fre- 



ture. Middle Devo- ,-, m i i i i 



quently a well-marked and perhaps 

 normal position for the shell, and 

 the necessity of keeping the mouth 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 capulid to Melocrinus JiieroglypMcus 

 Goldfuss and a smaller species to M. glohosus Fraipont. 

 These are from the Frasnian rocks of Senzeille, Belgium, 

 and have been figured by Fraipont.^ 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 ease mentioned in which the attached capulid spreads over 

 the side of the calyx, merely casual commensalism, but it is evident that the 

 shell covers the position of the anal orifice. 



