MORPHOLOGICAL PART. UB 
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which the brachials fill up the greater part, or all of the distal face of the 
radials, it required additional surfaces for the accommodation of the large 
tube. 
From the structure of the typical Poteriocrinidee we come to that of 
Ulocriuus, Graphiocrinus, Ceriocrinus, Erisocrinus, and Stemmatocrinus, which 
we regard as transition forms toward Encrinus. The ventral tube, which in 
the latter of these forms dwindled to a short cone, did not require as large a 
support in the dorsal cup, and as the anal plates gradually became obsolete, 
the posterior radials resumed a symmetrical form. In Udoerinus (Fig. 17) 
the plate z was crowded out by the large radianal; while in Graphiocrinus 
and Cerioerinus (Fig. 16) only the former is represented. In Graphiocrinus, 
with a wide ventral sac, the anal plate is large; in Ceriocrinus it is reduced 
to a small piece, and the posterior basal is considerably elongated. Eviso- 
cris (Fig. 18), Stemmatocrinus, and Encrinus have no anal plates at all, the 
cup being perfectly symmetrical; and the tube rests entirely upon the 
edges of the radials, whence it started in Hybocrinus, Ectenocrinus, and 
allied forms. 
In view of these facts, it seems to us that Bather’s theory of a “ brachi- 
anal” is based upon a wrong interpretation of some of the plates. If it 
were true that the plate of Jvcrinus to the left of the supraradial passed 
down in later forms to the basals, it would mean nothing less than a partial 
revolution of the entire tube. This, however, is disproved by the structure as 
well as the palontological development of the tube, which latter is generally 
composed of longitudinal rows of hexangular pieces, alternating in adjoining 
rows. In the earlier and simpler forms the tube consists of only five series, 
one to each interradius, that of the anal side resting upon plate ¢. Later on, 
as the tube grew larger, a new row of plates was introduced with plate z sup- 
porting it. When there are three series, as in Dendrocrinus, the third generally 
rests upon one side of the left posterior radial. The arrangement of the 
plates within the rows is so regular that if a sinking of the plate ¢ had taken 
place, it would certainly be indicated by some disturbance among the lower 
plates in the tube. In species where the tube has more than three rows, one 
or more of the primary rows dichotomize at some distance from the cup. 
The fact that the increase of the tube phylogenetically took place by the 
introduction of new rows of plates, is a strong argument in favor of our idea 
that the plate x is also a supplementary piece, and was introduced in the 
same manner as the plates which it supports. 
