454 Aug. F. Foerste 
38. The Thecal Plates of Cystaster and Hemic ystites 
In Hemicystites stellatus, Hall (pi. VI, figs. 6A, B), the inter- 
ambulacral and marginal plates are imbricating. In Cystaster 
granulatus, Hall (pi. VI, figs. 5A-D) the theca consists of a 
mosaic of minute polygonal plates, distinctly defined but irregu- 
larly arranged. These plates ought to show distinctly in speci- 
mens magnified to the extent of figure 5 A on plate VI, and should 
show even on the originals of figures 5B and C, if well preserved 
and thoroughly cleaned. In a specimen of cystaster granulatus, 
7.5 mm. in width, the diameter of the thecal plates varied be- 
tween one-fourth and one-third of a millimeter in the inter- 
ambulacral areas, and between one-fourth and one-fifth of a mil- 
limeter on the lower half of the thecal sac. The specimen 
appears to have been attached at the base. 
In Cystaster there is no indication of a peripheral ring. In 
Hemicystites carnensis, however, which is only an early form 
of Heynicystites stellatus, the weathered surface (lower part of 
fig. 2A on pi. Ill) clearly indicates the presence of large, nearly 
horizontal plates along the margin which represent the beginning 
of a peripheral ring; and in those specimens of Hemicystites 
which have the peripheral border extended, the smaller marginal 
plates are seen to be already differentiated. 
On the theory that the Agelacrinidae represent forms derived 
from some cystid source, Cystaster evidently presents the most 
primitive form, as pointed out by Bather. Judging from figure 
5D on plate VI, the form was only beginning to assume a sessile 
habit. Hemicystites, on the contrary, shows this sessile habit 
fully developed. Both forms appear closely related. Squa- 
mose imbricating plates are unknown among the true cystids; 
‘they can be regarded only as later developments from a primi- 
tive stock with a mosaic of polygonal, non-imbricating plates. 
Imbrication was due to dislocation in consequence of the species 
taking on a sessile habit. It probably originated at different 
times and in different lines of descent. Hence, the intimate 
relationship of Hemicystites to Cystaster, of Streptaster to such 
forms as Agelacrinus pileus (see fig. 1, on pi. II), and of Agela- 
crinites (fig. 3 on pi. VI) to Lepidodiscus (fig. 2A on pi. VI). 
