1881.] NATURAL SCIENCES OF PHILADELPHIA. 191 



the structure of the vault. In the genus Coccocrinus Miiller, 

 one of its earliest forms, the vault is composed of five large oral 

 plates, resting upon the upper truncate side of a single interradial, 

 and, as found in the fossil, it has a central oral opening and 

 lateral grooves for the ambulacral furrows. Zittel has already- 

 noted the close resemblance of the above structure with the recent 

 genus Hyocrinns Wj'ville Thomson, and the larval state of 

 Coma;!M?a, calling it very appropriately " ein embryonales Stadium 

 von Comatula in persistenter Form." The similiarity to Hyo- 

 crinus is probably merel}" superficial, as the lateral grooves in 

 Coccocrinus were evidentlj^ closed by additional plates as in other 

 Platycrinidae, while they are open in Hyocrinus. The oi'al plates 

 of Coccocrinus have been,b3' several authors, confounded with an 

 apparently similar superstructure in Symbathocrinus, Tnacrinus 

 and other forms, but there is really no analogy between the two 

 structures. The parts which enclose the opening at the oral 

 centre in the latter forms are radial in position, and therefore 

 not oral plates, but merely extended articulating facets of the 

 radials. In those genera, the central space, like the opening at 

 the centre of the oral plates, is also closed in perfect specimens 

 by apical dome plates, which rest directly upon the extended pro- 

 cesses. This group will be introduced hereafter as a separate 

 family under the name SymbathocrinidtB. 



The ventral disk in Coccocrinus bears a close resemblance to 

 that of Cyathocrinus, but while the former has an additional inter- 

 radial interposed between its radial and oral plates, in Cyatho- 

 crinus the intermediate plate is absent, and the oral plate rests 

 against the incurved upper margins of the radials. In Platycrinus 

 and similar genera, the two series of alternate plates which, as 

 mentioned before, cover the radial regions of the dome, are inter- 

 posed between three and sometimes five interradial plates, which 

 in Coccocrinus as oral and interradial plates occupy the same 

 position. This suggests the question whether these plates in the 

 Platycrinidae, and the interradial dome plates in the Sphteroido- 

 crinidoe generally, are not the homologues of the oral plates, which 

 are here broken up, and represented by several plates instead of 

 one. This interpretation seems to us the more probable because 

 Coccocrinus is one of the earliest known forms of the Platycrinidae, 

 and may be considered an embryonic type of the family. 

 The homology: in Platycrinus^ however, extends only to the 



