io6 



HISTORICAL PALEONTOLOGY. 



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

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



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

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

 Pseudocrinus bifasciatus, Upper Silurian, England ; D, Lepatiocrinns Gebhanii, Upper 

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



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- 

 mit of the calyx, and which are composed of innumerable 

 calcareo'us plates or joints, and are known as the " arms." In 

 the Cystideans, on the other hand, there are either no " arms " 

 at all, or merely short, unbranched, rudimentary arms. The 

 Cystideans are principally, and indeed nearly exclusively, 

 Silurian fossils ; and though occurring in the Upper Silurian 

 in no small numbers, they are pre-eminently characteristic of 

 the Llandeilo-Caradoc period of Lower Silurian time. They 

 commenced their existence, so far as known, in the Upper 

 Cambrian ; and though examples are not absolutely unknown 



* 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 Cvstidean. 



