616 Proceedings of the Royal Society 
two pairs of pores of each arc which are nearest the centre of the 
ambulacral area, pierce two small accessory plates intercalated be- 
tween the ambulacral plates, while the outer pair passes through 
the ambulacral plate itself near its outer extremity. The tube- 
feet on the oral surface of the body are provided with terminal 
suckers, supported by calcareous rosettes, while those on the apical 
surface are conical and simple. The tube-feet on both surfaces 
have their walls supported by wide cribriform calcareous plates. 
The peristome and the periproct are unusually large. The edge 
of the peristome is entire, without branchial notches, and the 
peristomial membrane is uniformly plated with twenty rows of 
imbricating scales, corresponding with the rows of plates of the 
corona, and the rows of ambulacral tube-feet are continued as in the 
Cidaridse, over the peristome up to the edge of the mouth. The 
ovarial plates are unusually large ; in some of the species they are 
broken up into several calcareous pieces. The ovarial apertures 
are very large, and are partly filled up with membrane. 
The dental pyramid is wide and strong, but somewhat low on 
account of the depressed form of the test. The epiphyses of the 
tooth-sockets do not form closed arches as in the Echinidae, and in 
this respect resemble those of Cidaris and Diadema. The teeth 
are simply grooved as in Cidaris. The spines are hollow and com- 
paratively small, and the larger spines show a tendency to the spiral 
arrangement of projecting teeth which is so characteristic of the 
Diadematidas. The Pedicellariae are very remarkable in form, 
more nearly related, however, to those of the Diadematidse than 
to any others. A strong fenestrated fascia traverses the body cavity 
vertically on either side of each ambulacral area, thus nearly 
cutting off the ambulacral from the inter- ambulacral region, and 
allowing only a small space for the coils of the intestine. 
For this family, distinguished by the depressed corona of imbri- 
cated plates, the peristome covered with scales through which the 
rows of ambulacral double-pores are continued to the mouth, the 
absence of branchial notches in the peristomial border, the 
peculiar arrangement of the ambulacral pores, the heterogeneity of 
the tube-feet on the oral and apical surfaces, the absence of closed 
arches uniting the pairs of tooth -sockets, and the absence of 
longitudinal ridges within the simple grooved teeth, the term 
