142 



ECHINODERMATA. 



CLASS OPHIUROIDEA. 



Ill the Ophiuroidea, tlie long slender serpent-like arms are 

 special organs of locomotion, being independent of the visceral 

 cavity ; and they are not grooved for tlie emission of tube-feet. 

 The "disc" contains all the vital organs. A masticatory appar- 

 atus is present in the inferior mouth ; but there is no anal aperture. 

 The madreporiform tubercle is inferior; according to some natur- 

 alists, the family holds the same relation to the Crinoids that the 

 Star-fishes hold to the Sea-urchins. 



The class is represented in tlie Upper Silurian b}' the genus 

 Pi'otaster. They become common in the Mesozoic and Tertiary. 



No. 436. [860, Cast]. Ophioderma Egertoni, Broderip. 



This genus differs from the OpMura proper in 

 having four (instead of two) genital fissures in 

 each interval between the arms. This specimen 

 is from the Lias of Lyme-Regis, England. 



CLASS CRINOIDEA. 



The Crinoids are among the most remarkable fossils that lie 

 entombed in the earth ; and it is only within the last century and 

 a half that their place in nature began to be understood. For 

 ages the superstitious or imaginative peasantry called them 

 " St. Cuthbert's beads," " rosary beads," "giants tears," "fairy 

 stones," "wheel-stones," "screw-stones," and "pulley-stones." 

 By early naturalists they were termed "Trochites," "Entrochus," 



