REPORT ON THE ASTEROIPEA. 469 



supradorsal membrane, situated over the centre of the disc and opening directly into the 

 dorsal cavity, to which it affords a common means of ingress and egress ; it is named the 

 oscular orifice. This aperture may be closed by five more or less regular fandike valves, 

 or simply by a number of webbed or papillose spinelets. A number of small apertures 

 open into the nidamental cavity on the actinal surface of the starfish, an aperture being 

 situated at the base of each of the long actinodateral spines and close up to the adambu- 

 lacral plate. There is consequently an opening into the cavity on each side of the furrow 

 corresponding to each segment of the ray ; hence these are spoken of as segmental 

 apertures. The openings are guarded and can be closed by a small spinelet or papilla 

 articulated on the adambulacral plate, and termed the aperture-papilla. In some genera 

 these appendages are partially hidden in the actinal membrane, and are free on one side 

 only ; in others they are perfectly free, and covered with a more or less expansive 

 investing membrane of their own. The long spines articulated on the body-frame close 

 to the adambulacral plates, which form the lateral or marginal web in Pteraster, and 

 support the whole actinal floor in Hymenaster, are designated the actino-lateral spines. 

 Finally, in the armature of the mouth-plates distinction is made between (l) mouth-spines 

 proper, which are situated upon the horizontal margins of the plates, and are usually 

 directed over the actinostome ; and (2) the secondary or superficial mouth-spines, which 

 are borne upon the surface of the plate, and usually stand perpendicular to its plane — 

 these latter spinelets being also frequently larger and more robust than the mouth-spines 

 proper. 



Synopsis of the Genera included in the Family Pterasterid.e. 



A. A supradorsal membrane present. "With actino-lateral spines. "With 

 segmental apertures. Form disco-pentagonal, rays slightly produced. 

 Abactinal spinelets long, forming highly developed pedicellated 



paxilla?. . • Pterasterin.b. 



a. Ai mature of the adambulacral plates forming transverse combs. Spines 

 united by web. 



a. Supradorsal membrane with muscular fibrous bands. Actino- 



lateral spines forming a free independent lateral fringe ; not 

 merged in the actinal floor, 

 a. Muscular bands not reticulated. Membrane usually con- 

 taining spicules . Pteraster. 



ft. Muscular bands regularly reticulated. No spicules in the 



membrane .......... Retaster. 



b. No muscular fibrous bands in the supradorsal membrane. Actino- 



lateral spines merged in the actinal floor. No lateral fringe. 



a. Paxillse-spinelets (fifteen to thirty), long and hair-like, protrud- 

 ing freely through the membrane. Adambulacral armatum 

 partly horizontal in disposition. One pair of secondary 

 mouth- spines ; united by web to the mouth-spine series . . Marm'patiar. 



ft. Paxillaa-spinelets (five or six), short, robust, not protruding. 

 Adambulacral armature perpendicular in disposition. 



