338 PALAEONTOLOGY OF ILLINOIS. 



In looking at the sides of this Platyceras, which has the form of a 

 very slightly arched cone,* and stands out nearly at right angles to the 

 side of the Orinoid, it is easy to see, from abrupt curves in the lines of 

 growth along up its sides, on a line above the sinuses mentioned, that 

 these sinuses commenced forming abruptly at points about half way 

 up ; and on measuring across between these points with a pair of divi- 

 ders, the space between is found to coincide very closely with that 

 between the inner sides of the arm-bases protruding from the sinuses. 

 Hence it is evident that the shell had commenced forming these sinuses 

 in its lip exactly at the period of its growth when it had attained a 

 breadth that brought the edges of its lip in contact with the arm-bases. 

 After this it had increased very little in breadth between the arms of 

 the Crinoid, though it had grown somewhat wider above and below, 

 and nearly doubled its length. Whether or not it covers the opening in 

 the side of the vault of the Crinoid we are unable to say, since the 

 folded arms (which are, as usual in these cases, well preserved) and 

 adhering matrix cover the vault. We have scarcely any doubt now, 

 however, that the Platyceras does in this, as in most of the other cases, 

 actually cover the opening in the side of the vault of the Crinoid. 



From the facts stated it is, we think, evident that these Mollusks 

 actually lived long enough after their connection with the Crinoids, to 

 which we find them attached, not only to have adapted the edges of 

 their lips to fit the surface of the Crinoid, but to have generally in- 

 creased more or less in size, and in some instances at least to have 

 actually nearly or quite doubled their size. Admitting this to be the 

 case and we think there can be no reasonable doubt on this point we 

 can no longer believe that these Crinoids were preying upon the Mol- 

 lusks, and we therefore think no well grounded arguments can be based 

 upon the fact of their being so frequently found attached in the manner 

 described in favor of the conclusion that the opening in the vault of 

 these Crinoids is the mouth. 



But, if they were not in the habit of eating these Mollusks, it may 

 be asked what could have been the nature of the relations between the 

 two that so frequently brought them together as we now find them f 

 The first explanation that suggests itself is, that possibly the Mollusk 

 may have been preying upon the Crinoid. But the fact, already stated, 

 that these Mollusks evidently lived long enough attached to these Cri- 

 noids, as we have every reason to believe, during the life of the latter, 

 to have at least increased the size of their shells considerably, if not 

 indeed during their entire growth, is alone an almost insurmountable 

 objection to such a conclusion. Doubtless, like other marine sedentary 

 animals, these Mollusks, when very young, floated freely about in the 



* It being the common species of Plati/ceras that is usually found attached to tins Plafycriiuix. 



