Affinities of Palceodiscus and Agelacrinus. 37 



features in the distinctness of lamellae through by far the greater 

 portion of its substance, and by the absence of secondary calcifications. 



TEXT FIG. 6. 



TEXT FIG. 7. 

 * 



TEXT FIG. 8. 



TEXT FIG. 6. Section through, tooth of Palceodiscus ferox I = lamellae. 



TEXT FIG. 7. Diagrammatic reproduction of cross section of an ambulacrum of 



Palceodiscus ferox ; o.s., outer series of plates ; i.s., inner series. 

 SXT FIG. 8. Diagrammatic reproduction of cross section of an ambulacrum of a 

 Lepidodiscus (Agelacrinus) pileus ; f.p., flooring plate; sc.p., side- 

 covering plate ; ag.amb.gro. = ambulacral groove. 



ie carina would seem, as in the other Echinoids, to be formed of 

 mse prisms. 



The Edrioasteroidea (Billings), or Thecoidea (Jaekel). 



History. The earliest member of this group to come under notice 

 was a form discovered by Dr. Bigsby in Canada and described by 

 Mr. G. B. Sowerby. Mr. Sower by compared it to " the arm of an 

 Asterias lying on an Echinoid," but felt himself unable to determine 

 its near affinities. 



Forbes, in 1848, described an English specimen of the same group 

 under the name of Agelacrinites Buchianus. Forbes remarks, "One 

 important point is strongly suggested by the structure of the arms 

 and arm grooves in this curious genus, viz., that it is possible the 

 ambulacral avenues of all Echinoidae are embodied arms, when embodied 

 usually separated by ambulacral plates ; that here we have the latter, 

 but no avenues, for the arms themselves are the avenues freed from 

 the body, consequently that the anomalous genus Palseechinus (and 

 perhaps Archseocidaris also) is an intermediate form between 

 Agelacrinites and true Urchins, an abnormal Agelacrinites, so to speak, 

 in which the arms have become embodied." 



Billings, in 1858, described two species which he calls Edrioaster 

 Bigsbyi and Agelacrinites Dicksoni. The ambulacral groove of Edrioaster 

 >i was composed of two series of oblong ossicles. Between the 



