Agelacrinidae and Lepadocystinae 
411 
of one plate to collapse within the proximal edge of the adjoining 
plate, especially along the margin of the sessile theca, at the pe- 
ripheral ring. Even in the interambulacral areas, the overlapping 
is always greater toward the peripheral ring than toward the 
mouth, as already stated. The imbrication probably began at 
the peripheral ring, and then progressed proximally to areas 
nearer and nearer to the mouth. 
In that case such species as the Devonian Agelacrinites hamil- 
tonensis may be regarded as merely forms in which the imbrication 
had not proceeded far from the margin of the thecal disc. 
In Streptaster, the interambulacral plates are small, polygonal in 
outline, and form a mosaic. In Streptaster vorticellatus , these 
plates are well known near the distal end of one of the interambu- 
lacral areas, where they are about half a millimeter in diameter, and 
form an irregular mosaic of polygonal plates. In Streptaster 
reversatus (plate IV, Fig. 3), the polygonal plates in the area 
between the left and left posterior rays are even less than half a 
millimeter in diameter, but in the posterior or anal interambulacral 
area, where the mosaic of plates consists of an irregular mixture of 
large and small plates, some of the larger plates attain a diameter 
of almost 1 mm. In all species of Streptaster, the plates forming 
the inner band of the peripheral ring are strongly imbricating. 
In Hemicystites the interambulacral plates are squamose and 
imbricating. In the closely related Cystaster, they are rounded 
or polygonal, and form a mosaic, although the individual plates 
are of minute size, averaging one-fourth of a millimeter in 
diameter. 
In Thresherodiscus (plate III, Fig. 3), the larger interambula- 
cral plates all are squamose and imbricating. The smaller plates, 
along the lateral margins of the rays, also overlap in a proximal 
direction, but in a less evident manner. 
Theoretically, the earlier forms of Agelacrinidae should show less 
imbrication among the interambulacral plates than the later 
forms. As a matter of fact, this is not verified by the specimens 
found so far. Both the imbricating and the mosaic type of inter- 
ambulacral plates occur in the Trenton, and the oldest species 
known at present have imbricating interambulacral plates. How- 
ever, none of these Trenton forms can be regarded as very 
primitive. 
