1869.] MEEK AND WORTHEN— ON PALEOZOIC CRINOIDEA. 451 



matrix, before the arraugement of the parts was clearly compre- 

 hended, these arms 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 Plafjjceras, 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 commenced 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 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 ai-e 

 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 

 Molluscs 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 surface 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 reasonable doubt on this point — we c:m no longer believe that 

 these Crinoids were preying upon the Molluscs ; 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. 



*It being the common species of Platyccras that is usually found 

 attached to this Platycrlnus. 



