REPORT ON THE ASTEROIDEA. 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 fan-like 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 actino-lateral 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 (1) mouwth-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. 
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 
paxille : . 5 : : : 4 : : 5 . Pr0rRASTERINA. 
a, Armature 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. 
8. Muscular bands regularly reticulated. No spicules in the 
membrane . : : ; 5 A A - ; : Retaster. 
b. No muscular fibrous bands in the supradorsal membrane. Actino- 
lateral spines merged in the actinal floor. No lateral fringe. 
a, Paxille-spinelets (fifteen to thirty), long and hair-like, protrud- 
ing freely through the membrane. Adambulacral armature 
partly horizontal in disposition. One pair of secondary 
mouth-spines ; united by web to the mouth-spine series . : Marsipaster. 
B. Paxille-spinelets (five or six), short, robust, not protruding. 
Adambulacral armature perpendicular in disposition. 
