GONIASTER. 



107 



2. Goniodiscus. — The marginal plates have the whole of theu' upper surface granulated. 



3. Stellaster. — The marginal plates are all granulated, and the ventral plates carry a 



suspended spine, as seen in the annexed figure, 34, of the under 

 surface of a ray of Stellaster CJiildreni} 



Fig. 34. 



Under surface of a ray of Stellaster Childreni, Grey. 



For our present purpose, I retain the genus Goniaster as originally defined, inasmuch 

 as the oolitic species hitherto discovered have not retained those delicate characters on 

 which the sub-genera of the ' System der Asteriden were founded. 



All the Goniasters have pentagonal bodies, with five angles, indicating the extremities 

 of the rays, which in some species project more or less. The disc is always flat in dried 

 specimens, or when removed from the water, but is capable of considerable elevation in 

 their native element. They however always want the con- 

 vexity of Asteropsis and the elevation of Oreaster. All 

 the species have their margins bounded by two rows of large 

 marginal plates (fig. 35 c), which enter into the formation of 

 the sides of the disc and arms, and are always larger than 

 the discal plates which occupy the upper and under surfaces 

 of the body (fig. 35 h), and (fig. 36 A and 5). 



The size and characters of the marginal plates render AstrogoniumLpidatum,u.^r.AT. 

 them, in a palgeontological point of view, the most im- 

 portant parts of the skeleton, as they are almost always well preserved, and afford, 

 at the same time, characters which appear to be very constant in the different 

 species. Their surface is either smooth or granulated, surrounded by granules or without 

 decoration, some having spines or pedicellarige, others being without such appendages. 

 The marginal plates enter into the formation of the border and form a firm frame-work, into 

 which all the other parts appear to be fitted (fig. 36). Among the living species, the 

 character of the marginal plates and their appendages is apparently much more constant 

 than among the fossil forms, and for this reason they have been used by zoologists for 

 classificatory purposes. 



See p. 47, for further details of these genera. 



