1002 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
dd. Abactinal skeleton stellato-reticulate, the papulae usually numerous and in definite areas 
(except in Asterodiscus) . Marginal plates sometimes superficially hidden. Abactinal 
plates often with large conical tubercles Pextacerotidas 
ddd. Abactinal skeleton tessellate; the plates often irregular and only partially contingent, the 
whole covered with a thick, leathery, skin. Flexible. Marginal plates inconspicuous, 
GYMNASTERIIDiE 
cc. Actinal interradial areas small. Superambulacral plates usually present. Pedicellarise, when 
present, excavate. Abactinal skeleton tessellate, arranged irregularly or in more or less 
regular longitudinal series, with intra- and often infra-marginal papulae. Rays usually 
slender, long, and subcylindrical Linckiida: 
aa. Marginal plates small and inconspicuous. Papulae not always confined to area circumscribed by 
the superomarginal plates, but often present between the marginal plates and on the actinal 
surface. Ambulacral plates may be crowded or not. 
b. Actinostomial ring with adambulacral plates prominent. Ambulacral plates not crowded. 
Abactinal skeleton composed of thin, close-set overlapping plates, or forming a more or less 
open reticulate network, either regular or irregular. In one family the abactinal plates are 
cruciform, with long paxillar spines supporting a supradorsal membrane. Pedicellarise very 
rare, never pedunculate or excavate. Tube feet biserial, 
c. Mouth plates small, ambulacral groove narrow. Spines not conspicuously long and slender, or 
supporting a supradorsal membrane. 
d. Abactinal skeleton is formed of closely imbricating plates bearing small spines. Actinal 
skeleton formed of imbricating plates bearing a tuft or fan of spinelets. Marginal plates 
minute. 
e. Papulae distributed throughout the abactinal area. Abactinal plates thick, crescentiform, 
devoid of internal processes Asterixidas 
ee. Papulae confined to the radial regions. Abactinal plates in the median regions stellate. 
Abactinal plates thin, scale-like, with elongate internal prolongations. General form 
very thin and flat Anseropodida: 
dd. Abactinal skeleton formed of plates disposed in longitudinal and transverse series, or in an 
irregular network, bearing spinelets; spinelets not disposed in a tuft or fan. 
e. Spinelets small, pointed, naked, or covered with a thin skin containing calcareous granu- 
lations. ( Valvaster has large marginal bivalved pedicellarise) Echinasteridjs 
ee. Spines heavy, rigid, obtuse, covered with spiniform scales. Reticulation of the dorsum 
formed of triangles grouped in hexagons Mithrodiidse 
cc. Mouth plates conspicuously large, plowshare-shaped, with conspicuous mai-ginal and actinal 
spines. Marginal plates not visible. Abactinal plates cruciform as a rule, bearing a 
fascicle of rather long delicate spinelets united by a fold of the integument, or supporting 
a supradorsal membrane which roofs a specialized nidamental cavity. 
d. No supradorsal membrane. Spines united by a web. Rays more than 5 Myxasterida: 
dd. A supradorsal membrane present . Pterasteridas 
bb. Pedicellarise pedunculate, either forficiform or forcipiform (composed of 2 jaws and a basal 
piece). Skeletal plates bearing spines, often long and isolated, on or about which are 
usually grouped the pedicellarise; or the pedicellarise may be isolated. Marginal plates 
inconspicuous or aborted. Ambulacral plates often very crowded so that the tube-feet 
are disposed in 4 series. Abactinal skeleton formed of skeletal arches, independent or 
bound together by intermediate plates, forming a network with rectangular or very irregular 
meshes; rarely in mosaic. These skeletal arches correspond to every other, or to every 
third adambulacral, and are composed of pieces corresponding in the ventral, lateral, and 
dorsal regions of the body. Mouth plates very inconspicuous as a rule, 
c. Tube-feet quadriserial, at least at the base of rays. Actinostome with ambulacral plates 
prominent. 
d. More than 25 arms; interbrachial septa double Heliasteribas 
dd. Less than 15 arms (Hawaiian species, 5-armed) ; interbrachial septa single. 
e. Adambulacral plates of 2 kinds, alternating, one projecting into the furrow, and separating 
with its spinelets the tube-feet; the other not prominent. They bear several spinelets in 
a transverse series. Skeleton regular, composed of imbricating plates. . .Zoroasterid.®; 
