8 



ASTEROIDEA. 



tesselated inter-ambulacral plates forming the floor of the rays, with the narrow ambulacral 

 ossicles and the stellate mouth-opening. 



^' In Astrogonium magnijicum, M. and T., the superior border- 



plates are encircled two thirds by rows of granules ; and the 

 upper surface of the arm is covered with large circular or oblong 

 plates, smooth and convex on their upper surface, and each 

 surrounded by a complete circle of granules. This structure 

 , . , is exhibited in the annexed figure 9, from Miiller and Troschel. 



Upper surface of a ray of o 

 Astrogonium magnijicum, M. and T. 



In the genus Stellaster, the upper and under sides of the pentagonal body are flat, and 



surrounded by two rows of large marginal plates, both 

 of which enter into the formation of the high border ; 

 each of the lower marginal plates carries, near the 

 outer side, a flat, moveable spine (fig, 10) ; and several 

 granules are scattered over the surface. The ambu- 

 lacra are narrow, and the suckers biserial ; both sides of 

 the intra-marginal. disc are covered with granulated 

 plates, on which numerous pedicellariae are fixed. Fig. 

 10, which exhibits the under surface of one of the rays 

 in Stellaster Childreni, Gray, illustrates the characters of this genus. 



Fig. 10. 



Under surface of a ray of Stellaster 

 Childreni, Gray. 



Ftg. 11. 



In Ojphidiaster the rays are long, cylindrical, or conical ; the osseous framework of 

 each consists of a series of ossicula of two or three different forms ; in this section of a ray 

 (fig. 11) there are seven rhomboidal ossicula, of which three belong to the upper surface, 



and two to each of the sides ; eight 

 oblong ossicula {a) unite the rhom- 

 boidal pieces {h) together, two uniting 

 the rhomboidal ossicles of the upper 

 surface of the ray with each other, two 

 linking the superior lateral with the 

 surface plates, two connecting the pairs 

 of lateral rhombs of each side (b), and 

 two articulating the inferior laterals with 

 the small, round ossicles which link the inter-ambulacral plates {a) with the long femur-like 

 ambulacral ossicula projecting upwards into the interior of the arm (fig. 11 a). In this 

 transverse section of a ray of an Ophidiaster we find nineteen ossicula, of which four or six 

 belong to the ambulacral area, and the others to the inter-ambulacral portion. If the 

 ossicula of an arm, therefore, were folded down, and extended outwards, the six ambulacra 

 would form the centre, and the other plates on each side would represent the two halves 

 of the adjoining inter-ambulacral areas. 



c a i 



Section of a ray of Ophidiaster, with the inter-amhulacral plates. 



