94 POTEEIOCRIMDiE.— STNBATHOCEIKUS. 



The dorso-central plate of the S. conicus resembles that of a Platycrinus, as do also 

 the perisomic plates, which consist of one series only, as is common to that genus, but 

 the manner in which the rays articulate is totally different in the two genera. In the 

 Platycrini, as may be seen on reference to our illustrations, they invariably articulate by 

 a lunate excavation in the radial plates, but in the Synbathocrinus they are attached by 

 the whole breadth of the plates. The structure of the rays, which are long, differs but 

 slightly from that of several species of Poteriocrinus. 



The small interradials which we have termed meso plates, and which are so character- 

 istic of the Platycrines are wholly wanting in this genus ; nor would the form of the 

 lateral plates admit of the introduction of meso plates. In the Platycrini the marginal 

 angles at the summit of the plates are cut off, forming an angular notch between every 

 two plates for the introduction of the meso plates. In Synbathocrinus the lateral plates 

 have no retiring angles in which meso plates could possibly be inserted. 



Only a few specimens of this small but interesting fossil have been discovered. Mr. 

 Gilbertson, we believe, developed two or three specimens without rays or columns, from 

 the carboniferous limestone of Yorkshire ; Mr. Morgan was so fortunate as to discover 

 at Barry Island, the specimen from which fig. 5 b, pi. 11, was drawn, and the Earl of 

 Enniskillen has since obtained from the Hook Point limestone, the specimen represented 

 inPl. ll,fig. 5 a. This specimen has been deposited in the Bristol Scientific Institution. 



It is a remarkable circumstance that of all the varied forms in which the Crinoidea 

 have appeared during different geological epochs the number of pieces forming the 

 dorso-central plate has never exceeded five. All the gradations from one to five prevail, 

 but that number has never been exceeded. In this, and Uie genus Platycrinus, we see 

 it whole and undivided, in the Dichocrines it is composed of two pieces, in the 

 Hexacrines the number three prevails, in Cariocrinus and Tetramerocrinus four is the 

 dominant number, in Cyathocrinus and some other genera it is composed of five pieces, 

 but at this point further subdivision ceases. 



