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 agglotneratci, of the natural size ; d, Vertical section of two tubes of the 

 same, showing the tabular, enlarged. Niagara Limestone (VVenlock), 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 (Ecliinoids} are said 

 to be represented by examples of the old genus Palczchinus. 

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

 are, comparatively speaking, largely represented , the former 

 by species of Palasterina (fig. 60), Palaaster (fig. 60), Palao- 

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

 latter by species of Protaster (fig. 61), Palceodiscus, 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. 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 



