114 



ANNULOIDA. 



The ossicles are united with one another in a reticulated man- 

 ner, and the interspaces between them are filled by the coria- 

 ceous integument. In some genera there is a single or double 

 row of large plates round the borders of the disc and arms 

 (fig. 66, m). These are termed the " marginal plates." The 

 integument in the Star-fishes is also furnished with spines, 

 tubercles, and granules of calcareous matter. The spines 

 vary in their development and in their position in different 

 Star-fishes ; but there is commonly a row of spines along each 

 side of each of the ambulacral grooves. In some genera (as 

 in Solaster, Luidia, Ctenodiscns, &c.) there are spines the 

 summits of which carry bunches or tufts of minute calcareous 

 processes. These are termed " paxillae." Lastly, in Asteroidea, 

 as in Echinoidea, there are the modified pincer-like spines 

 which are known by the name of " pedicellariae." 



As regards their geological distribution, the Asteroidea have 

 a long vertical range, extending from the Lower Silurian Rocks 

 to the present day. In Silurian seas Star-fishes abounded, 

 and the Upper Silurian Rocks especially contain their remains 

 in what may be considered as plenty. The leading Silurian 

 genera are Palaaster, Stenaster, Palasterina, Palceocoma (Salter), 

 Palaodiscus, and Petraster. Some of the more familiar Silurian 

 forms are figured below (fig. 68). The next period in which 



Fig. 68. Silurian Star-fishes. A, Palasterina primerva. Upper Silurian ; B, Pala- 

 aster Ruthveni, Upper Silurian ; C, PaUeoconta Colvini, Upper Silurian 

 Salter.) 



(After 



Star-fishes abound is the Oolitic or Jurassic (Mesozoic), the 

 more important genera being Uraster, Luidia, Astropcctcn, and 

 Goniastcr. All the Oolitic species are extinct, but the genera 

 have mostly survived to the present day. In the Cretaceous 

 period, also, many Star-fishes occur, the genera Orcaster, Go- 

 niodiscus, and Stellaster being the most noticeable. In the 

 Tertiary Rocks remains of Star-fishes are not abundant, but 

 the genera Goniaster and Astropecten are represented in the 

 London Clay (Eocene). 



