HETEROCRINIDAE 
91 
Lateral arms as in C; inferradials of vanished 
arms meet under x; segments of 1 . ant. R always sepa- 
rated. BB 3, the fused left basal more or less curved, 
not touching stem. Devonian to Lower Carboniferous. 
Halysiocrinus. 
Lateral arms 2, balanced, with axil-arms borne 
on short faces of successive axillaries, arching to- 
ward anal tube; inferradials of vanished arms sepa- 
rated by subanal piece; segments of 1 . ant. R usually 
separated. 3 BB, 1 . ant. and 1 . post, fused to a trian- 
gular plate, not touching stem. Silurian. Calceo- 
crhius. 
R. ant. arm vanished, leaving median and 2 lat- 
eral arms balanced ; stem shifted to plane of anal 
tube; segments of 1 . ant. R narrowly connected ; 4 BB, 
all touching stem. Silurian. Eucheirocrinus. 
Crown bent upon stem ; r. post, arm and basal 
vanished, leaving median and 3 lateral arms, un- 
symmetric, anal tube, and 4 BB all touching stem; 
stem at side of tube; segments of 1 . ant. R broadly 
connected. Ordovician to Silurian. Cremacrinus. 
Probable ancestral form like Heterocrimis, with 
5 arm-bearing RR, 2 simple, 3 compound; crown 
erect upon stem ; anal tube springing from r. post. R. 
Ordovician. Dotted line indicates arm and basal first 
to be eliminated. 
Fig. I. The Series of Calceocrinidae 
Read from below up 
