SUMMARY. 



185 



Genus 28.— Pluaiaster, Wric/ht (Vol. II, p. 112). 



Body stellate, rays numerous, elongate, expanded in middle, provided on under 

 side with a single row of marginal plates, transversely elongate, slightly arched, 

 pectinated on one border, and displaying a single row of tubercles carrying long spines. 

 Ambulacral furrow narrow and depressed. Radial ossicles at the angle of the rays very 

 prominent and sculptured. Two rows of ambulacral pores. Genus found in the Liassic 

 strata. One British Liassic species (Vol. II, p. 112). 



Genus 29. — Astropbcten, Linck (Vol. II, p. 113). 



Body stellate, depressed, rays five, elongate. Margin of rays provided with two 

 rows of marginal plates, those on under side bearing spine-like scales, which increase in 

 size from within outwards, and terminate in long spines ; those on upper side covered 

 with granules and often becoming spinous. Upper surface of body and rays (apart 

 from the marginal plates) clothed with granules or plates, which are crowned with 

 groups of minute spines. Ambulacral furrows more or less broad. Two rows of 

 ambulacral pores. Range of genus from the Lias to existing period. Thirteen British 

 Liassic and Oolitic species (Vol. II, pp. 113 — 129, 168). 



Order III.— OPHIUROIDEA, Wright (Vol. I, p. 5; Vol. II, p. 131). 



Body {disc) discoidal, depressed, having long slender arms {rays), destitute of an 

 excavation for the viscera. Upper surface of disc supporting at base of arms ten large 

 calcareous pieces [radial ^^lates), sometimes close together, sometimes entirely naked, 

 sometimes partly clothed with the general covering of the body. Under surface of disc 

 bearing large smooth pieces {buccal plates) of different forms, generally single, and ten in 

 number, sometimes doubled, and consisting of an inner larger and outer smaller portion. 

 Under surface of disc also marked in each of the angular spaces between the arms {inter- 

 brachial) by genital openings [fissures], either two or four, or ten. Mouth {oral 

 opening) always central, on the under side provided with a masticatory apparatus consisting 

 of a cone of calcareous pieces, and always serving for the anal opening. Skeleton of many 

 calcareous pieces variable as to number, size, and arrangement, and covered with an 

 integument, either naked or bearing granules, scales, or spines. Rays simple or ramified, 

 enclosed in four sets of jointed calcareous plates, one above, one below, and one on each 

 side ; the lower set single, double, or quadrupal ; spines developed on the sides ; under 

 surface not grooved and not pierced for tubular feet. Madreporiform tubercle on under 



