154 PEor. p. M. duncak's eevision or the 



Peristome deep, its margin composed of ten ambulacral and five 

 interradial plates, two splieridia in each ambulacrum hidden. 

 Straight grooves ia the ambulacra, and buccal pores at the margin 

 of the peristome. Teeth, jaws, and perignathic girdle as in 

 Glypeaster. Periproct at or close to the margin. 



Internal structure without concentric partitions or false walls 

 at the edge, or separate needles, expanded above and below near 

 the edge, but with the petaloid jjarts of the ambulacra more or less 

 limited by stout dif continuous septa or pillars which extend some 

 way towards the apex, and are curved around the poriferous zones ; 

 the ambulacral and intestinal areas imperfectly separated. The 

 ambulacra, with the exception of the poriferous zones, have an 

 inner wall, produced by the coalescence of broad calcareous 

 growths from the inner surface of the plates remote from their 

 transverse sutures, and wbich, after reaching a certain height, 

 expand and unite so as to form a wall standing upon low broad 

 foundations ; the raised foundations bound, and with the expan- 

 sion form, the canals for the main and secondary water-systems 

 of the ambulacra ; foramina occur along the median line and 

 near the poriferous zones. This inner wall supports expansions 

 of pillars and ueedles. 



Ornamentation small ; tubercles sunken and surrounded with 

 miliaries. Spines short, slender. Pedicella-riae, some very large, 

 tiidact}le and tumid-headed. Two hidden spheridia in each 

 auibulacrum. 



I'ossil. Tertiary : West Indian Islands (Anguilla) ; San 

 Domiugo. 



Eecent. "West Indian Islands, Plorida. (Littoral to 5 fathoms.) 



Genus Plesiais^thus. 



Syn. Glypeaster, Lamk. (pars) ; Echinantlius, Gray, 1855, 

 A, Agass, 1872-4, pars. 



Test with a tumid, subpentagonal margin, tumid and depressed 

 dorsally, concave actinally ; peristome deeply seated. 



Apical system with a button- or star-shaped central madre- 

 porite ; basal plates fused ; genital pore beyond and in the 

 median suture of the interradium; radial plates very small, pore 

 circular or elongate. 



Ambulacra witli the petaloid parts long, tumid, broad, closed, 

 or nearly so ; poriferous zone broad, sloping or depressed, pores 

 unequal, distant ; plates compound, the adoral component a 



