REPOKT ON THE ASTEROIDEA." 465 



Adarnbulacral plates with an armature forming with that of the infero-marginal plates 

 (and actinal intermediate plates if present) a transverse series of large, single, isolated 

 spinelets, invested with membranous sacs. (Six are present in transverse series in the type 

 form.) 



Mouth-plates with a prominent median keel. Armature consisting'of : (1.) A marginal 

 series of six mouth-spines on each plate, the innermost one much larger than the others ; 

 all invested with membrane. (2.) On the actinal surface of the plate is one large 

 secondary mouth-spine in a membranous sheath, placed immediately behind the large 

 innermost marginal mouth-spine. 



Madreporiforrn body very large and irregularly oval in outline, the margin being 

 festooned. Surface grooved with numerous fine striations. 



Ambulacra! furrows wide. Tube-feet large and crowded, forming four rows ; with a 

 fleshy, button-like, terminal disk. 



No pedicellarise of any kind present. 



Remarks. — This handsome and well-characterised form is undoubtedly allied in many 

 points of structure to Korethraster. Peribolaster is, however, at once distinguished by 

 the reticulate abactinal skeleton, by the cruciform plates composing it, by the sheathed 

 fascicules of the abactinal plates, by the presence of papulae on the abactinal surface, by 

 the extraordinarily large madreporiforrn body, and by the quadruple series of ambulacral 

 tube-feet. 



Chorology of the Genus Peribolaster. 



a. Geographical distribution : — 



Pacific : One species between the parallels of 45° and 50° S. 



Peribolaster folliculatus, off the west coast of Patagonia. 



/3. Bathymetrical range : 45 fathoms. 



7. Nature of the Sea-bottom: Green sand. 



Chorological Synopsis of the Species. 



1. Peribolaster folliculatus, n. sp. (PL LXXIII. figs. 4-7). 



Rays five. R = 54 mm. ; r = 18 mm. R = 3 r. Breadth of a ray at the base, 

 about 20 mm.; and midway between this and the extremity about 17'5 mm., measured 

 outside the spinelets. 



(ZOOL. CHALL. EXP. — PART LI. — 1888.) 59 



