230 pEor. p. M. Duncan's eevision of the 



a fifth basal, two, three, or all perforated by ducts ; the madre- 

 porite in the right anterior basal or extending centrally and more 

 or less posteriorly, separating the postero-lateral basal plates and 

 sometimes the posterior radial plates also. 



Ambulacra diverse, the anterior in the groove; the pores 

 oblique, in pairs on either side, plates numerous and low ; antero- 

 lateral ambulacra petaloid dorsally, sunken, more or less tending 

 to close, diverging ; the postero-lateral ambulacra with shorter 

 petals than the antero-lateral or subequal, not so divergent ; the 

 pores of the petaloid parts of both, of the ambulacra subequal, 

 sometimes diverse, usually in narrow grooves, the outer pores 

 usually the larger, elongate ; actinally the posterior ambulacra 

 are broad on either side of the tumid sternum, the pores beyond 

 the petals distant but in pairs. 



Interradia narrow at the apex and also at the peristome, with 

 but few plates, many tumid ; a small labrum anterior to an 

 amphisternum ; the episternum not developed ; the plates below 

 the periproct subequal. Normal heteronomy of right posterior 

 interradium by union of the second and third plates of zone a. 



Peristome excentric in front, transverse, open, semi-lunar, with 

 a posterior labrum and often with raised antero-lateral edges. 

 Periproct ovoid or elliptical, long vertically, high in the posterior 

 truncation. Ornamentation small, a few primary tubercles per- 

 forate and crenulate, closest and largest actinally near the 

 margin and on the sternum ; miliaries abundant. 



A peripetalous fasciole encircling the petaloid parts of the 

 ambulacra, crossing the anterior groove and the keel behind the 

 vertex, angular or subcircular, variable in its entry between the 

 petals. Large disciferous tentacles within the fasciole on the 

 anterior ambulacrum, others more or less branchial on the 

 petals. Spines small, largest actinally and at the edges of the 

 petals, which they may cover over. Depth of ambulacral petals 

 variable according to age and sex, used as marsupia in the females. 



Type I. EtJimopJiracti. — With four perforated basal plates, the 

 madreporite in the right anterior basal plate, and not 

 separating the other basal plates. 



Type II. Ethmolysii. — Two or three, rarely four, basal plates 

 with genital ducts ; the madreporite passing centrally and 

 posteriorly, separating the postero-lateral basal plates and 

 in some the posterior radial plates also. 



