450 THE CANADIAN NATURALIST. [DeC. 



self-evident that the coutinuation of the life of the latter must 

 have necessarily been of vei*y short duration after it came in 

 contact with the Crinoid. Yet we have the most conclusive 

 evidence that such was not the case ; but that on the contrary, in 

 most of, if not all of these instances, the Platyceras must have lived 

 long enough in contact with the Crinoid to have adopted 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 

 specimens of Piatt/ crinus hemispha;ricus, and Goniasteroidocrinus 

 tuberosus, 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 Crinoid to be clearly seen, the sinuosities of the lip of the 

 former closely conform to the irregular nodose surface of the 

 latter. Owing to the fact that in some cases the shell has 

 evidently been forced by accidental pressure against the surface 

 of the Crinoid, 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 Platyceras 

 attached, as usual, to its side, between the arm-bases of two of 

 its adjacent rays, and of rather larger size than those usually 

 found attached to this species, the adaptation of the irregularities 

 of its lip, so as to receive the little nodes and other prominence 

 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 occupies 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 lip for the reception of these arms. These sinuses are not 

 only in precisely 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 Mollusc had attained its present size, without either 

 breaking its lip or breaking ofi" the arms of the Crinoid. 

 Unfortunately, in clearing away the rather hard argillaceous 



