BELASTER ORDOVICUS. 73 



Genus BELASTER, novum. 



Generic Characters. — Arms long, straight-sided, with no great differentiation 

 between the proximal and median marginalia. Radialia stout, almost square. 

 Mouth-angle plates large and conspicuous. 



Only one species is known, Belaster ordovicus, n. sp. 



]. Belaster ordovicus, n. sp. Plate II, figs. 6, 6a; Plate III, fig. 3; Text-fig. 36. 



Material. — One specimen in the collection of Mrs. Gray shows both the 

 imprint (D. 44) and the counterpart (D. 44c). 



Apical Surface (Plate II, figs. 6, 6rt ; Text-fig. 36). — The cast shows one arm in 

 full length, the bases of two other arms and scattered ossicles of the disc region. 

 The arm has three rows of ossicles ; the radialia bordered by two rows of supero- 

 marginalia. All these plates are strongly tumid. The proximal radialia are 

 somewhat crushed together by post-mortem change. 



The most proximal radial is somewhat rounded, whilst the three following 

 radialia are approximately square. The diagonal of the square runs along the 

 main axis of the arm. Each of the sides of the square is indented. All these 

 radialia lie alternately with the supero-marginalia. From this point the radialia 

 become elongated and rounded, and tend to lie opposite to the supero-marginalia, 

 persisting through the entire length of the arm. At the extremity of the arm 

 there is a distinct ocular shown more distinctly in the fossil than in the photograph 

 of the cast. 



The supero-marginalia are very conspicuous, and certainly not subordinated to 

 the infero-marginalia (see above, p. 6Q). They are finger-shaped almost through- 

 out the length of the arm. The most proximal pairs of the plates meet across an 

 interradius and form a broad margin to the disc. The second supero-marginalia 

 meet these proximal supero-marginalia at a very obtuse angle, and from this point 

 the margin is continued almost as a straight line throughout the arm. 



A side view of the arm shows that the marginal wall is high and that the 

 supero- and infero-marginalia distinctly alternate. At the extremity of the arm the 

 infero-marginalia as usual pass over to the apical surface. There are five infero- 

 marginalia corresponding to the distal three supero-marginalia. 



Oral Surface (Plate III, fig. 3). — The mould on the counterpart gives a cast 

 of only a portion of the arm, photographed in PI. Ill, fig. 3. The measurements, 

 form, and arrangement of the infero-marginalia and the adambulacralia are very 

 similar to those of the specimen described below from the Ruddy Collection. 



Ornament. — The plates are covered with granule-like eminences. These are 

 seen clearly on PI. II, fig. Qa. 



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