336 PALAEONTOLOGY OF NEW YORK. 



S. scintilla, S. Keokuk) which have usually been found attached to flatter sur- 

 faces. 



Mr. Etheridge makes the following interesting observations on these 

 fossils : 



" When the organism to which the Productus is attached is of larger size 

 than the latter, the whole of the ventral valve is applied to it, the spines 

 spreading out and around on each side ; but when the foreign body is of less 

 diameter than the Productus, as is usually the case with fragments of Polyzoa, 

 several of the spines are wound tightly round, especially near the beak, and 

 the remainder of the valve remains free. Attachment took place during the 

 life of the Crinoid ; for in nearly every case where the Productus remains 

 adhering, we find that its rate of growth was less than that of the Crinoid, the 

 result being that the substance of the latter surrounded or enclosed its parasite, 

 first the encircling spines disappearing and gradually the shell. We have 

 specimens showing this remarkably well in all stages of the process, from the 

 mere absorption of the spines by the substance of the Crinoid, up to the total 

 disappearance of the Productus itself, when the Crinoid stem assumes a swollen 

 or distorted appearance. From a consideration of this gradual absorption by 

 the Crinoid stem there arise two questions : Did the Productus when once 

 attached lose the power to free itself ? or, Did the absorption by the Crinoid, 

 contrary to the view indicated previously, commence only after the death of 

 the Productus?" 



Some of Mr. Ethertdge's figures of this species, which is from the Lower 

 Carboniferous limestone of Edinburghshire, Scotland, are here reproduced. 



