THE LOWER SILURIAN PERIOD. 



107 



to note that many of the calcareous deposits of the Lower 

 Silurian are strictly entitled to the name of " Crinoidal lime- 

 stones, " being composed in great part of the detached joints, 



Fig. 46. Group of Cystideans. A, Caryocrinus ornatus,* Upper Silurian, America; 

 B, Pleurocystites squamoaus, showing two short " arms," Lower Silurian, Canada; C, 

 Pseudocrinus bifasciatua, Upper Silurian, England; D, Lepudocrinus Oebhardi, 

 Upper Silurian, America. (After Hall, Billings and Salter.) 



and plates, and broken stems, of these beautiful but fragile 

 organisms (see fig. 12). Allied to the Crinoids are the singular 

 creatures which are known as Cystideans (fig. 46). These are 

 generally composed of a globular or ovate body (the "calyx"), 

 supported upon a short stalk (the "column"), by which the 

 organism was usually attached to some foreign body. The 

 body was enclosed by closely-fitting calcareous plates, accu- 

 rately jointed together; and the stem was made up of numerous 

 distinct pieces or joints, flexibly united to each other by mem- 

 brane. The chief distinction which strikes one in comparing 

 the Cystideans with the Crinoids is, that the latter are always 

 furnished, as will be subsequently seen, with a beautiful crown 

 of branched and feathery appendages, springing from the sum- 



* The genus Caryocrinus is sometimes regarded as properly belonging 

 to the Crinoids, but there seem to be good reasons for rather considering 

 it as an abnormal form of Cystidean. 



