120 



HISTORICAL PALAEONTOLOGY. 



Amongst the Echinodermata, all those orders which have 

 hard parts capable of ready preservation are more or less 



d 



Fig. 59. a, Halysites cntennlaria, small variety, of the natural size ; b, Fragment of 

 a large variety of the same, of the natural size ; c, Fragment of limestone with the tubes 

 of Halysites agglomerata, of the natural size ; d, Vertical section of two tubes of the 

 same, showing the tabulae, enlarged. Niagara Limestone (Wenlock), Canada. (Original.) 



largely represented. We have no trace of the Holothurians 

 or Sea-cucumbers ; but this is not surprising, as the record of 

 the past is throughout almost silent as to the former existence 

 of these soft-bodied creatures, the scattered plates and spicules 

 in their skin offering a very uncertain chance of preservation 

 in the fossil condition. The Sea-urchins (Echinoids} are said 

 to be represented by examples of the old genus Pa/echinus. 

 The Star-fishes (Asteroids] and the Brittle- stars (Ophiuroids) 

 are, comparatively speaking, largely represented , the former 

 by species of Palasterina (fig. 60), PalcRctster (fig. 60), Palczo- 

 coma (fig. 60), Petraster, Glyptaster, and Lepidaster and the 

 latter by species of Protaster (fig. 61), Palaodiscus, Acroura, 

 and Eucladia. The singular Cystideans, or " Globe Crinoids," 

 with their globular or ovate, tesselated bodies (fig. 46, A, C, D.), 

 are also not uncommon in the Upper Silurian ; and if they do 

 not become finally extinct here, they certainly survive the close 

 of this period by but a very brief time. 13 y far the most im- 

 portant, however, of the Upper Silurian Echinoderms, are the 

 Sea-lilies or Crinoids. The limestones of this period are often 

 largely composed of the fragmentary columns and detached 



