PENTAGONASTER OBTUSUS. 75 



Specific Characters. — Disc slightly convex. Majority of the plates covering 

 the disc of snbequal size and closely set. R : r : : 25 mm. : 1"2 iinn. Extremities 

 of arms obtuse. Interbrachial arcs only slightly rounded, giving the disc a 

 distinctly pentagonal appearance. 



Material. — The two extremities of the arms from which Forbes originally 

 described the species when in the Dixon collection, are now preserved in the 

 British Museum (Natural History). They are not on the same slab of chalk as 

 represented by Forbes, but are and probably always have been two independent 

 specimens, E. 5038 (PI. XXII, figs. 3 h, c,<l), and E. 5039 (fig. 3 (/). More complete 

 specimens have since been added to the collection (40400, PI. XXII, fig. 1 ex 

 J. Simmons' Coll., and 35481, PI. XXII, fig. 2, e.v. H. W. Taylor's Coll.). Two 

 extremities of arms ai-e also preserved in the Brighton ]\Iuseum. 



Descnption. — Tlio disc is high and distinctly pentagonal. At the edge of the 

 disc the dorsal covering plates are crowded and jioh^gonal in appearance. Towards 

 the centre they become slightly smaller and rounded. The average diameter of 

 these plates is 1"7 mm. 



The madreporite is subcentral in position. It is about the same size as the 

 other plates of the disc and is pentagonal in shape (PL XXII, fig. la). 



The arras are stoutly built. A triple row of polygonal plates enters their 

 bases. The adradial series soon disappears, leaving the single radial series, which 

 appears to persist until it reaches that part of the ray which is obtuse. From this 

 point the supero-marginal plates may or may not be adjunct up to the end of the 

 ray. Considerable variation appears to exist as to this point in the single 

 specimen examined. The arms are distinctly obtuse in their distal half. This has 

 given the species its name. 



The supero-marginalia form a rather broad Ijorder to the disc. There 

 are nine supero-marginals from the median interradius to the extremity of the 

 ray. Towards the end of the ray the plates of this series become narrower, more 

 oblong in appearance, and distinctly convex. 



The infero-marginalia are equal in number and situated generally alternating 

 with the supero-marginal series. They are distinctly square in shape, especially 

 at the obtuse extremities of the ray. Their ornament is in some specimens not 

 so coarse as that of the supero-marginals. 



The actinal interradial areas are very distinct and are occupied by four series 

 of ventro-lateral plates. As usual, the actinal plates near the mouth are larger and 

 more rhomboidal than the distal plates. Ventro-lateral plates only extend 

 throughout about a quarter of the length of the arm. 



12 



