170 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
brachials six-sided, pointed at both ond>^. Type, Siluraster 
perfedus Jaekel. Upper Ordovician, Bohemia. 
Another starfish which has the simple arrangement of plates 
on the ventral surface characteristic of the Hudsonasteridae 
is Palaeaster caractaci Gregory. This species is so much like a 
"Hudsonaster" that Spencer^ at first identified it with Proto- 
palaeaster narrawayi. Both Schuchert and Spencer now place 
it in the Mesopalaeasterinae and Spencer has erected a new 
genus, Caractaster, for it, considering it the most primitive 
member of the family in which it is now associated. The family 
characteristic (two rows of adradial plates) is, however, con- 
fined to the abactinal side, and only by its longer and more 
slender arms can it be distinguished from one of the Hudson- 
asteridae, if the actinal side only is preserved. The inter- 
brachial area is still represented by only a single plate the outer 
end of which forms a part of the marginal series, and which is 
nearly oval in shape, pointed outwardly. The shape of this 
plate is probably of generic value, for it is evidently of a form 
which represents the last stage before the interbrachial is 
pushed inward from the margin. 
The ventral side of Palaeaster is also very like that of the 
species here described, but fortunately the plates of the only 
known species of that genus are rather distinctly marked. 
Each of the infra-marginals has a large, smooth central space 
encircled by a pustulose area, which makes them easily recog- 
nizable. 
From the foregoing synopsis of the characteristics of the 
ventral surface it appears that if only that side of the speci- 
men is preserved, one has to depend largely upon the shape of 
the interbrachial plates for identification of genera, taken, 
however, in connection with all other characteristics which can 
be observed. The diagrammatic figures given in Plate 3 (fig. 1-6) 
serve to illustrate the variations in outline. All are seen from 
the oraF.side, with the inner end downward. 
'Palaeontographical Soc, London, vol. for 1913, 1914, pp. 21, 30. 
