STRUCTURE OF THE RAYS. 



9 



Fig. 12. 



In Arthraster the number of the ossicula is less, and their arrangement diflFerent, than 

 in Opliidiaster. This genus, which is cretaceous and extinct, has, according to Forbes, 

 only seven ossicula in the framework of the ray, 

 as shown in the transverse section, fig. 12 ex- 

 clusive of the ambulacral bones, which are unknown. 

 These plates are articulated together in such a 

 manner as to form a compact armature sur- 

 rounding the arms (fig. 12 a), like the plates and 

 sutures in the test of Echinidce, of which they 

 are the homologues. All the seven ossicles are 

 similar in form, each consisting of a transversely oblong, expanded, linear base, 

 terminating in an acute angle at each end (fig. 12 h), and bearing along the centre a crest- 

 like ridge with steep sides. The central ossicle is the largest, and this Forbes regarded 

 as the equivalent of all the ossicles in the upper surface of the arm of Opliidiaster (fig. 11), 

 whilst the others may be considered as the homologues of the lateral and ventral plates, 

 with their connecting ossicula.' 



Section of a ray of Arthraster, with the side 

 plates. 



Fig. 13. 



The genus Oreaster comprises a group of pentagonal star-fishes, which have the under 

 surface flat, and the upper surface more or less elevated ; large tubercles or globular 

 calcareous spines occupy various parts of the dorsal surface (fig. 13) ; the skeleton is formed 

 of large plates, mostly of an irregular polygonal 

 shape, which are disposed on the ridges of the arms 

 in a more or less squamated order ; the margins of 

 the rays are surrounded by two rows of granulated 

 plates, which overlap each other, the dorsal border- 

 plate alone forming the margin, and the ventral 

 border-plate lying on the under surface. There are 

 two rows of tubular feet in each avenue, and a sub- 

 central vent on the dorsal surface, which is elevated 

 and sub-pyramidal (fig. 14). The interior of the 

 test is strengthened by calcareous pillars, as in the 

 Cli/peasters among the Echinid^. The valve-like 

 pedicellariaj are all sessile. 



Upper surface of Oreaster turritus, Linck. 



1 'Memoirs of the Geological Survey,' vol. ii, part 2, p. 467. 

 of Sussex,' p. 336, pi. 23, fig. I. 



See also Dixon's 'Geology and Fossils 



2 



