238 THE CRINOIDEA -CAMERATA OF NORTH AMERICA. 
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municates with the perforation in the arms; axillary plates well developed ; 
arms round and generally set at right angles to the body.’ He took the 
upper openings, 7. ¢., the central perforation following the median line of the 
appendages, to be “efferent tubes,” carrying off the water used for respira- 
tion, which he thinks in other groups is performed by the anal tube. 
From numerous specimens in our collection, some of them with all of the 
two kinds of appendages preserved to their full length, we are enabled 
to fully confirm the opinion of Meek and Worthen that the smaller, pinnule- 
bearing appendages are arms, and that the stouter, tubular ones are struct- 
ures unlike those of other Crinoids. What the functions of these tubes may 
have been can only be conjectured, but they were probably not identical 
with those of the cirri, as Meek and Worthen supposed; and we are inclined 
to think, from the fact that their canals communicate with the subtegminal 
galleries at the inner floor of the ventral disk, that the functions, as sug- 
gested by Grenfell, were respiratory, and that the canals performed a similar 
office to that of the respiratory pores of Sa/ocrinus and the spiracles of the 
Blastoids. 
Meek and Worthen, in separating the European species from the Ameri- 
can, were probably not aware that the coalesced appendages are actually 
pairs of distinct tubes, each one having a canal of its own, and the canals 
of the same pair communicating with different ambulacra, in a way similar 
to that of the paired spiracles in the Blastoid genus Pentrenutes. It is in this 
respect exceedingly interesting that in Gilbertsocrinus jiscellus the posterior 
appendages are simple, and are widely separated by a row of anal plates, 
exactly as in the European species; while those of the four other sides are 
united at the base. The species thus represents at two of its sides the 
European form of the genus, and at the other three the American. 
Another interesting fact in the developmental history of the genus is 
that in the species of the Burlington limestone the consolidated tubes are 
composed of the single cylindrical plates of two simple tubes united laterally 
by suture. In the transition beds between the Burlington and Keokuk 
groups appears the rare species G. obovalus, in which the two rows of plates 
composing the double tubes of its predecessors are roofed over by four more ; 
and this character becomes constant in the Keokuk, where the genus is 
found quite abundant in some localities. 
The genus Phodocrinus, which has been so frequently confounded with 
Gilbertsocrinus, has arm openings only, the tubular appendages being un- 
represented ; otherwise the two genera cannot be distinguished. 
