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644 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
ay 
D.C. Dorsal cup. 
V.D. Ventral disk. 
b. basals. 
R. radials, 
I. costals. 
ar. interradials (in part interbrachial, in 
part interambulacral). 
ia.  interambulacrals of the regular sides. 
we.  interambulacrals of anal side. 
0. orals. 
cp. covering plates. 
bal 
Platycrinus. 
feemarks. —The Platycrinide are the typical family of a sub-group which 
is distinguished from all other families of the Camerata by having the brach- 
: lals and interbrachials only slightly represented in the dorsal cup. The 
| lower brachials, although incorporated into the calyx, belong in part to the 
cup and in part to the disk, and have more or less the form of free arm 
plates. From this we conclude that the Platycrinide represent phylogenet- 
ically a less developed stage of the Camerate Crinoid, in which the lower 
arm plates remained in an immature condition, their small size giving rise 
to the interposition of three plates to fill up the interradial space in the first 
row, instead of the usual single one. The lower interradials, therefore, are 
for the most part neither strictly interbrachial nor interambulacral, but may 
be considered as belonging to both. This is not invariably the case, however. 
In a remarkable undescribed species of Platycrinus from the Mountain lime- 
stone of England, the costal and the two distichals are connected with those 
of adjoining rays by five or six heavy plates, which are strictly interbrachial 
