64 Dr. T. Wright on Fossil Echinoderms 



have dedicated this species to the Earl Ducie, who collected the 

 beautiful specimens we have figured. 



Family CLYPEASTRID.E. 



This natural family includes all the Urchins which have a 

 circular, elliptical, or pentagonal form, with a thick test, the 

 surface of which is closely covered with small, nearly equal- 

 sized tubercles sunken in the plates, and surrounded by ring- 

 like areolas ; these all carry short hair-like spines. The mouth 

 is large, central and pentagonal, and is armed with five strong 

 ,.. jaws which carry the same number of teeth : the anus is pos- 

 W terior, and marginal or inframarginal : the interior of the test 

 is sometimes divided by pillar-like processes of the inner layer 

 of the plates. The dorsal portions of the ambulacral areas have 

 a petaloid form, circumscribed by large poriferous zones ; the 

 basal portions are narrow, rectilineal, or branched. The five 

 genital plates form a circle around the madreporiform body, 

 and between these are wedged the five ocular plates. This 

 family includes the genera Clypeaster, Lamk., Laganum, Klein, 

 Echinarachnius, Van Phels., Arachnoides, Klein, Scutella, Lamk., 

 Dendraster, Agass., Lobophora, Agass., Encope, Agass., Rotula, 

 Klein, Mellita, Klein, Runa, Agass., Moulinsia, Agass., Scutellina, 

 Agass., EchinocyamuSy Van Phels., Fibularia, Lamk., Lenita, 

 Desor. 



Genus CLYPEASTER (Lamarck, 1816). 



Form oval, inclining to pentagonal, rostrated before, truncated 

 behind ; upper surface more or less inflated, sometimes campa- 

 nulate, conical or subconical ; inferior surface flat, always concave 

 around the mouth, with five straight simple ambulacral furrows 

 proceeding from the angles of the mouth to the border; the 

 dorsal portion of the ambulacral largely petaloid, greatly exceed- 

 ing the interambulacra in size, and forming elegant leaf-like 

 expansions, in general convex, arched, and prominent ; bounded 

 on each side by large poriferous zones, the pores of which are 

 wide apart and united by transverse sulci ; the apical disc formed 

 of five genital plates at the summits of the interambulacra, with 

 five ocular plates alternating with them ; in the centre of this 

 circle is the spongy madreporiform body, of a pentagonal figure : 

 tubercles uniform in size and very numerous, equally distributed 

 over the test; summits perforated, and surrounded by very 

 deep areolas ; mouth symmetrical, central, pentagonal, lodged in 

 a concave depression in the middle of the base ; auricles composed 

 of ten distinct auricular processes set in pairs : the jaws form 

 a pentagonal pyramid, composed of ten separate pieces, trun- 

 cated at the summit, which is bordered by a subcircular band ; 



