No. 16122. ABNORMAL ARM STRUCTURE IN CRINOIDS—CLARK. 269 
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of Decametrocrinus from that found in Promachocrinus. In L[elio- 
metra, as in Antedon bifida, the only plate separating the radials in 
the young is the anal, and it is possibly to the resorption of this plate 
and the subsequent filing up of the resultant gap that the torsion 
of the calyx in Promachocrinus is due; there is no such torsion in 
Decametrocrinus,; wight not the inference be made that in the young 
a similar plate occupied a// the interradial areas? We get this state 
of affairs in Vhawmatocrinus, which I have already shown resembles 
the Pentametrocrinide (Pentametrocrinus and Decametrocrinus) im 
arm structure; and it may well be that the young of Pentametro- 
erinus and Decametrocrinus will be found to be very similar to, or 
identical with, Thawmatocrinus. - 
Mr. Springer has recorded a number of abnormal arm structures 
in Uintacrinus socialis, and it is interesting to compare these with — 
‘similar cases among recent comatulids. In Utntacrinus the lower 
pinnules are arranged somewhat differently from what they are in 
most comatulids; the first is on the outer side of the second joint from 
the costal axillary, the next on the inner side of the fourth joint, then 
on the outer side of the fifth, the inner side of the seventh, and then 
alternately on every joint (except hypozygals of syzygial pairs). 
This is not such an anomalous arrangement as it may seem at first 
sight, for it really means that the first two Joints beyond the costal 
axillary are distichals, not true brachials, the joint corresponding to 
the distichal axillary merely bearing a pinnule, not another arm. 
This is similar to the case in Hudiocrinus (as restricted), in which 
genus the costal. “axillary” bears a pinnule instead of the second 
arm. The true free arm in Uintacrinus, therefore, starts at the third 
joint from the costal axillary, this being in reality the first free 
brachial; the fourth and fifth free brachials (sixth and seventh joints 
from the costal axillary) appear to form the first svzygial pair. In 
most comatulids the third and fourth free brachials constitute the 
first syzygial pair; but it need not surprise us to find the first brachial 
syzvyey in Uintacrinus one joint farther on, as in its nearest relatives, 
the Comasteride, the position of the first brachial syzygy is, NECOrdine 
to species, benveen the first and second, second and third, or third Wha 
fourth brachials, while in the Pentametrocrinide@ the first syzygy is 
between the fourth and fifth free brachials, just as in Uintacrinus. 
Considering the first two joints beyond the costal axillary as in 
reality distichals, we should expect to find that occasionally an arm 
was developed here, instead of the more common pinnule; and Mr. 
Springer (Plate v1, fig. 9) figures a specimen in which this condition 
occurs. The left arm of this distichal pair has a pinnule on the first 
joint beyond the distichal axillary, but this is due to the omission of 
the preceding joint; this joint corresponds to the axillary of a v palmar 
series, the free brachials beginning at the next joint. 
