244 J. Rofe — Notes on the Crinoidea. 



were also enveloped in, and connected to eact other by, a similar 

 membrane, and not by sutures, and were so far analogous to the 

 Echinidan described by Dr. Carpenter. When the test is entirely 

 calcified, as is mostly the case when in solid rock or encrinal marble, 

 the joints of the plates have a minutely wrinkled surface, probably 

 due to the junction of the membranes which have become calcified, 

 and apparently form part of the substance of the plate, and these, 

 being in actual contact, woiild not suggest the idea of any power of 

 modifying the form of any part of the calyx. But specimens are 

 occasionally found, some of which are in my collection, which, like 

 the screw-stones, are represented only by a siliceous pseudomorph of 

 the membrane surrounding the plate, from which the calcareous part 

 has been removed. In these cases the membrane retains its shape 

 and separate existence, and where any of the calcareous nucleus 

 remains, as is sometimes the case, it may easily be separated by 

 dilute acid, frequently leaving the external form of the fossil and 

 the outer covering and ornamentation of the plates perfect. The 

 heads generally have the appearance of being much weathered and 

 partially decayed empty tests, but still having the external cha- 

 racters so visible that the genus, and not uncommonly the species, 

 may be distinguished. Some of them had been in my possession 

 some time, when Mr. Parker, of Manchester, well known in the 

 district for his knowledge of Bolland and Clithero fossils, called 

 my attention to the fact of their being siliceous, as they would 

 scratch glass. On this hint I made some experiments which 

 justified Mr. Parker's suggestion, and combining this with the 

 siliceous screw-stones of which I was before aware, it appeared a 

 fair inference that, in the one case as in the other, the animal 

 membrane was replaced by silica and thus apparently preserved, 

 whilst the calcareous portion of the echinoderm had been dissolved 

 and removed, and that the plates of the head as well as of the 

 column were united by membrane, and not by suture, and were 

 consequently flexible. 



Specimens of Actinocrinus and Bhodoct'inus have been found in 

 this state, and Fig. 9, Plate VI., shows a portion of a Bhodocrinus in 

 which sections of the membranous envelopes of some of the plates 

 are seen in contact, but the calcareous centres or cores have been 

 dissolved out. 



Detached plates of an Echinidan from the Mountain Limestone 

 are not unfrequently found near Clithero, which are also enveloped 

 in a siliceous case, and in many instances, although detached, a con- 

 siderable number of plates are found so close together as to have the 

 appearance of being part of an Echinidan flattened out by pressure. 



That the enveloping membrane should be sometimes calcified, as 

 it undoubtedly is in encrinal marble and in limestone generally, and 

 at other times silicified, as in the chert and in the cases under con- 

 sideration, is a subject which will admit of more than one hypothesis 

 equally applicable to fossils in Chalk and flint, but which has no im- 

 mediate connexion with the present question. 



Miller, who, in his work on the Crinoidea, assumes the proboscis 



