54 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



vegetable remains is embedded. Fragments of petrified wood occur also in the Lias, 

 having large groups of My till, in the position that is usually assumed by recent 

 mytlli, attached to floating wood. 



I have examined similarly preserved material, and attached to one 

 bit of wood not more than 15 inches in length by 3 inches in diameter 

 there were to be found 20 or more Pentacrinus. Walther has brought 

 forward similar evidence relative to the fossil Pentacrinidse. It is 

 probable in such cases that the attachment was not one of cementa- 

 tion. Although the nature of the association could not be accurately 

 determined in the material examined, it seems highly probable that 

 attachment was had by means of the radicular cirri. Walther con- 

 siders the crinoids to have wound their stems about the drifting wood, 

 as may well have been the case in some instances. As he shows, the 

 attachment was so firm that frequently the crinoids were carried up 

 into brackish or fresh water embayments, where they were deposited 

 in the coal beds then forming. In such cases the driftwood with the 

 appended crinoids may well have been driven in by storms. 



It may be argued that in the cases above cited the wood to which 

 the crinoids were attached was not floating at the surface, but had 

 become water-logged and sunk to the bottom. Indeed, it is hard to see 

 how the crinoids primarily became attached to the wood, providing the 

 latter were at the time freely floating. Although one may concede 

 considerable freedom of motion to the vagrant stalked crinoid, it does 

 not seem probable or possible that the forms could swim freely at the 

 surface. The mero-planktonic larvse apparently oijer the most fea- 

 sible solution of the problem, but here another factor must be con- 

 sidered, and that is the length of time wood will float. Obviously in 

 the present instance the period would have to be of sufficient length 

 to permit of the maturation of the crinoid from an early larval stage. 

 The arguments presented by Buckland and quoted above seem fairly 

 conclusive that the wood with its pendant crinoids did float at the 

 surface. The evidence brought forward by Walther likewise tends 

 to prove that the crinoids were true epi-plankton. Were this not 

 the case, it would obviously be impossible for the crinoids to have 

 been driven into the brackish-water deposits where they were found. 

 If this be true, it seems probable that the animals had passed the 

 greater portion of their lives so attached. 



Plankton: Scyphocrinus. — Unquestionably the widest deviation 

 from the normal habit of the stalked Crinoidea is to be found in the 

 case of the genus Scyphocrinus. For a crinoid to assume the role of a 

 vagile benthos is not remarkable, and indeed one is not surprised to 

 find an epi-planktonic existence sporadically maintained, as in the 

 case of Pentacrinus. To find a stalked crinoid acquiring such struc- 

 tures as enable it to maintain a truly planktonic existence, however, 

 is a most anomalous condition of affairs. Nevertheless, such we find 

 to be the case in Scyphocrinus. 



