ASTEEOIDEA OF H.M.S. ' CHALLENGER ' EXPEDITTON. 221 



spinelets are placed slightly oblique, aud are covered with an 

 investing membrane which unites them together at the base and 

 joins them up to the neighbouring pairs, concealiug to a certain 

 extent their actual size and shape. 



Mouth-plates moderately large and prominent, sharply up- 

 turned along the line of junction, forming a swollen, semitu- 

 bercular keel considerably elevated above the general surface of 

 the ventral area. A single mouth-spine is borne at the mouth- 

 angle, placed on the line of suture and directed inwards ; and 

 two smaller, compressed, subtriaugular mouth-spines stand on 

 the margin of each mouth -plate near their aboral extremity. No 

 secondary mouth-spines are present on the surface of the mouth- 

 plates. 



The peristome is wide, occupying fully two ^fifths of the 

 ventral area of the disk ; and the mouth is situated in the centre 

 of a naked leathery membrane and furnished with a well- 

 developed muscular lip. 



The interbrachial areas form a fairly regular equilateral 

 triangle — the distance from the innermost point of the mouth- 

 angle to the place where the area terminates and the marginal 

 plates join up immediately to the adambulacral plates being 

 about halfway out on the ray, and approximately equal to the 

 base of the area that extends along the arm-angle. The area is 

 covered with a compact pavement of calcareous plates, sub- 

 hexagonal or subquadrate in form, arranged in columns parallel 

 with the median interradial line. The whole is covered with a 

 thick membrane, through which the plates are scarcely per- 

 ceptible until the specimen is dried. 



The madreporite is large and conspicuous, subcircular or oval 

 in form, and situated in an interradium close up to the dorso- 

 marginal plates, and consequently adjacent to the cribriform 

 organ. Sometimes it is subtriaugular in contour, with the base 

 directed to the margin, and the striae form lines sharply bent at 

 an angle, running more or less parallel with the sides of the 

 triangle, the open angle being directed towards the base. 



Colour, in alcohol, yellowish white, with a bluish-grey shade over 

 the disk. The marginal plates and the actinal interbrachial areas 

 have a peculiar glazed appearance, consequent on the character 

 of the investing membrane. 



Station 45. Lat. 38° 34' N., long. 72° 10' W. Depth 1240 fms. ; 

 bottom temperature 2°'4 C. ; mud. 



