ae 
ea 
Ei. Billings on the structure of the Crinoidea, ete. 53 
~ having been discovered in the primordial zone. No other echin- 
oderms have been found in rocks of so ancient a date. 
Next in order may be placed those genera whose test is com- 
posed of a definite number of plates, which have, to some ex- 
tent, a quinary arrangement. Thus, Glyptocystites, Hehinoen- 
crinites, Apiocystites and several others, have each four series of 
calycine plates, of which there are four plates in the basal and 
five in each of the other three series. The respiratory areas or 
hydrospires are reduced in number—ten to thirteen in Glypio- 
Fr : 
Chazy limestone and seems to have become extinct in the Tren- 
n. The other genera occur in various horizons between the 
Chazy and the Devonian. 
| These fi ups represent the five ambulacral canals of 
the recent estos es the specimen from which this dia- 
