FOSSILS OF THE BUKLINC.TOX GROUP. 337 



life of the latter must have necessarily been of very short duration after 

 it came in contact with the Crinoid. Yet we have the most conclusive 

 evidence that such was not the case : but on the contrary, in most if not 

 all of these instances, the Platyceras must hace lived long enough in con- 

 tact icith the Crinoid to }xive adapted' the sinuosities of the margins of its 

 shell exactly to the irregularities of the surface of the Crinoid. 



We have taken some trouble to examine carefully a number of speci- 

 mens of Platycrinus hemispha>ricu$. and Goniaxtcroidocrinus tuberosw, 

 from Crawfordsville. Indiana, with each a Platyceras attached, and in 

 all cases where the specimens are not too much crushed or distorted, or 

 the hard argillaceous shaly matter too firmly adherent to prevent the 

 line of contact between the shell and the Crinoid to be clearly seen, the 

 sinuosities of the lip of the former closely conform to the irregular no- 

 dose surface of the latter. Owing to the fact that in some cases the 

 shell has evidently beeu forced by accidental pressure against the sur- 

 face of the Criuoid, so as to become somewhat crushed, this adaptation 

 is not always so clearly evident : but in most cases it is more or less 

 visible, while in some it is strikingly manifest. In one instance of a 

 Platycrinus now before us, with a Platyctras attached, as usual, to its 

 side, between the arm-bases of two of its adjacent rays, aud of rather 

 larger size than those usually found attached to this species, the adapta- 

 tion of the irregularities of its lip, so as to receive the little nodes aud 

 other prominences of the Crinoid, is so clearly manifest that a moment's 

 examination must satisfy any one that the shell must have grown there. 

 Being, as we stated, a larger individual than we usually see so situated, 

 it not only occupied the whole of the interradial or anal space to which 

 it is attached, but its lateral margins on each side coming in contact with 

 the arm-bases of the Crinoid, as the shell increased in size, had formed on 

 either side a profound sinus in its Up for the reception of tliexe arms. These 

 sinuses are not only in precisely the proper places, but of exactly the proper 

 '(nd form to receive the adjacent arm on each side, the entire adjust- 

 ment 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, without either breaking its lip or breaking 

 off the arms of the Criuoid. Unfortunately, in clearing away the rather 

 hard argillaceous matrix, before the arrangement of the parts was 

 clearly comprehended, 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 extend- 

 ing up from the edge of the lip. 



44 



