106 HISTORICAL PALAONTOLOGY. 
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 sguantosus, showing two short ‘‘arms,” Lower Silurian, Canada; C, 
Pseudocrinus bifasciatus, Upper Silurian, England ; D, Lepadocrinus Gebhardi, 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 
calcareous 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 Cr7znoids, but there seem to be good reasons for rather considering 
it as an abnormal form of Cystidean. 
