Plate 76. 

 Fifis. 1-5. Siderolik's tclraedra. 



1. Young, ventral view. >( 45. 



2. Later stage, ehanilx'i's invadinii- (Idvsa! side. X W. o, venlral side; ft, dorsal 



side. 



3. Still later stage whi'ic dorsal side is ])artly covered hy tlie invading cluinibers. 



X 35. a, ventral view; b, dorsal vi<nv. 



4. Old eroded specimen, in which the chaniljers are larucly broken away, leaving 



the raised bosses ix) as projections from the center. X ll^. 



5. Specimens in which the cluunbcr^ now cover even the spines. X 15. 



565 



