968 THE VOYAGE OF H.M.S. CHALLENGER. 



The Zygostephauida, constituting the first subfamily, exhibit four large gates only, 

 and differ from all other Coronida in the absence of a basal ring and of basal gates. 

 Therefore the skeleton is entirely composed of two crossed vertical rings, perpendicular 

 to one another; the first is the primary or sagittal ring (inherited from the Stephanida), 

 the second is the new lateral or frontal ring. The four large lateral gates are either quite 

 simple [Zygostophanus) or partly closed by loose and irregular lattice-work {Zygo- 

 stephanium). The Zygostephauida may be derived directly from the Stephanida by 

 development of lateral branches forming a frontal ring. They commonly possess the 

 same characteristic spines or branches, and the same typical difi"erence between the straight 

 dorsal rod and convex ventral rod of the sagittal ring, which we found in the greater 

 number of Stephanida. The frontal ring is commonly elliptical or kidney-shaped, and 

 much larger than the ovate sagittal ring. 



The Acanthodesmida, forming the second subfamily of Coronida, differ from aU other 

 members of this family in the possession of a large simple basal gate, surrounded by a 

 simple horizontal basal ring. Only this ring is complete, whilst the two crossed vertical 

 meridian rings (the primary sagittal and the secondary frontal ring) are incomplete, 

 both truncated at the base by the basal ring. Therefore there remain here between the 

 three rings five large gates (recognised previously by Johannes Miiller in 1856 inAcantho- 

 desmia) : four lateral gates (the same as in the Zygostephauida) and one central basal 

 gate. The latter is always quite simple, and serves for the emission of the pseudo- 

 podia, arising from the basal pole of the central capsule. The fovir lateral gates 

 are either quite simple {Coronidium) or partly closed by irregular loose lattice-work 

 {Acanthodesmia). The subfamily Acanthodesmida may be derived either directly from 

 the Stephanida (by development of a central basal gate) or from the Eucoronida (by 

 loss of the basilar part of the sagittal ring). 



The Eucoronida, the third subfamily, are the most important group of the Coronida ; 

 their numerous species are much more frequent and more widely distributed than those 

 of the other three subfamilies. They may be derived immediately from the Semantida 

 by the development of a lateral or frontal ring. This remains incomplete in the basal 

 part, whilst the two other rings, perpendicular to it (the vertical sagittal ring and the 

 horizontal basal ring), are comj)lete. Therefore the shell constantly exhibits six large 

 open gates between the three rings ; four lateral gates (the same as in the Zygo- 

 stephauida and Acanthodesmida) and two basal gates (inherited from Semantis) ; the 

 latter correspond to the " jugular pores " of the Spyroidea and Cyrtoidea; they 

 remain constantly simple. The four lateral gates may also remain simple {Eucoronift) 

 or they may become partly closed by irregular loose lattice-work {Plectocoronis). The 

 remarkable genus Podocoronis is distingiiished by the development of typical descending 

 basal feet, which are regularly disposed and correspond to the typicfd " cortinar feet " of 

 the other Nassellaeia. There may be developed either two lateral feet (as lower 



