12O 



HISTORICAL PALEONTOLOGY. 



Amongst the Echinodermata, all those orders which have 

 hard parts capable of ready preservation are more or less 



Fig. 59. a, Halysites catennlaria, small variety, of the natural size ; 6, Fragment of 

 a large variety of the same, of the natural size; c, Fragment of limestone with tie 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 Pal&chinus. 

 The Star-fishes (Asteroids) and the Brittle- stars (Ophiimnds) 

 are, comparatively speaking, largely represented , the former 

 by species of Palasterina (fig. 60), Pal&aster (fig. 60), Paltzo- 

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

 latter by species of Protaster (fig. 61), Pal&odiscus, Acroura, 

 and Eudadia. 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 bi'ief time. By 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 



