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NEW YORK STATE MUSEUM 



attachment between Platyceras and the crinoids. With the 

 opening of the Devonic the development of Platyceras became 

 enormous, so much so that the calcareous phase of the earliest 

 Devonic has been termed the Platyceras stage. The crinoids alsc 

 were common at this time, but cases of any dependent conjunction 

 of the two are extraordinarily rare ; the only instance of this earl}; 

 date known to me is that cited by Drevermann from the Cob- 

 lentzian. Little by little, however, the habit was assumed and 

 becoming more frequent in the Middle Devonic it seems to have 

 attained a culmination in the faunas of the earlier Carbonic. 

 During all the ages which have intervened between the 



Silicified specimens of Platyceras attached to the dome of Megistocrinus farns- 

 worthi White, from the Middle Devonic of Iowa. The perfect adjustment of the shell 

 to the crinoid is seen in the adaptation of its margin to every irregularity of the surface 

 of the dome. Loaned by Prof. Samuel Calvin 



Paleozoic and the present there is no record which has come to 

 my notice to prove that this ancient habit has had an uninter- 

 rupted existence. The crinoids and the limpets have continued 

 and certainly the detailed records of Mesozoic and Cenozoic faunas 

 should have given some account of this habit had it perdured. 

 We have remarked that the consociation was always an easy one 

 to which even at its hight not all the members of the genus Platy- 

 ceras \Yere compelled. In the absence of demonstration, it may be 

 fair to hold it possible that the descendants of these mollusks really 

 abandon"fed this form of attachment and rebounded from the degen- 

 erative condition which it involved; this would be a fact of pro- 

 found significance if it indicates that an organism once started on 

 the downward path can take a new hold of life and regain its inde- 

 pendence. Yet we are doubtless not justified in such a conclu- 



