NATURAL SCIENCES OF PHILADELPHIA. 333 



cisely the proper places, but of exactly the proper size and form to receive the 

 adjacent arm on each side ; the entire adjustment being so exact, that it 

 seems scarcely possible that the shell could have been removed during the 

 life of both animals, and after the Mollusk had attained its present size, with- 

 out either breaking its lip or breaking off the arms of the Crinoid. Unfortu- 

 nately, in clearing away the rather hard argillaceous matrix, before the 

 arrangement of the parts was clearly comprehended, these arras were broken 

 away, but their stumps are still seen protruding from the sinuses, which are 

 so deep as almost to present the appearance of isolated perforations, though 

 it is evident, on a careful examination, that they are only deep emarginations 

 extending up from the edge of the lip. 



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 

 Crinoid, 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 com- 

 menced forming abruptly at points about half way up ; and on measuring 

 across between these points with a pair of dividers, 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 bad 

 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 breadih 

 between the arms of the Crinoid, though it had grown somewhat wider above 

 &nd he\ow,a.nd nearl;/ doubled lis lenffth. Whether or not it covers the open- 

 ing 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 open- 

 ing in the side of the vault of the Crinoid. 



From the facts stated it is, we think, evident that these MoUusks 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 lip to fit the sur- 

 face of the Crinoid, but to have generally increased 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 reas- 

 onable doubt on this point — we can no longer believe that these Crinoids 

 were preying upon the MoUusks; 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 MoUusks, 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 ? The first explana- 

 tion that suggests itself is, that possibly the Mollusk may have been preying 

 upon the Crinoid. But the facf, already stated, that these MoUusks evidently 

 lived long enough attached to these Crinoids, as we have every reason to be- 

 lieve, 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 al- 

 most insurmountable objection to such a conclusion. Doubtless, like othfr 

 marine sedentary animals, these Mollnsks, when very young, floated freely 

 about in the sea, until they found a suitable station to attach themselves, 

 where they remained during life. May they not, therefore, have been attracted 

 to the bodies of Crinoids by the numerous little organisms brought in by the 

 action of cilia, along the ambulacral furrows of the arms of the Crinoids, or 

 in currents produced by the motions of the arms of the latter? The excre- 



* It being the common species of Platyceras that is usually found attached tothis Platy- 

 crinus. 



1868.] 



