O34 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
the inner sides. They are succeeded in full grown specimens by two series 
of plates of the same order, which to the fifth or sixth rows are incorporated 
into the calyx. There are ten pairs of very large arm openings directed 
upwards, of which those of the same pair are contiguous, while those of 
adjoining pairs are placed apart, being separated by well marked depressions. 
In young specimens, the arms are free above the second distichal, and there 
are but five pairs of arm openings, which are arranged as the ten in the 
older ones. Between these two forms there are others of intermediate stages 
in which, although having as yet but ten openings, these are arranged 
singly — not in pairs —and their axillary distichals form the uppermost 
plate of the ray in the calyx. Arms long, rather stout, frequently dichot- 
omizing. Interbrachials in large specimens: 1, 2, 2, 3, 4, 4, 2, with slight 
variations in the upper rows ; interdistichals, 1, 2, 8, 2; anal interradius: 1, 
3, 4, 0, 7, and a number of smaller plates above. The smallest specimen 
under examination has but 1, 2, 2 interbrachials, a single very minute inter- 
distichal, and 3, 4, and 5 plates above the first anal. Tegmen low hemi- 
spherical to almost flat, with distinct plications toward the outer margin 
— corresponding to the rays and their main divisions — and a slight groove 
at the anal side. Posterior oral highly convex, conical, or even spinous, and 
sometimes as large as the four others together, from which it is separated by 
several rather large, tumid plates, and in the larger specimens by very small, 
irregular, flat pieces of subsequent growth interspersed between the larger 
ones. The smaller orals and radial dome plates are surrounded by similar 
plates, which increase in number, as well as in size and convexity, with the size 
of the specimen. In the smallest examples before us there are five minute, 
isolated pieces, interposed at the ends of the inter-oral sutures, and the orals 
are still in contact among themselves and with the radial dome plates by 
small surfaces. In the next largest specimens, the interposed plates, although 
yet very small and flat, are united laterally so as to separate the orals, as 
well as the radial dome plates. In the largest specimens, the interposed 
plates are not only larger but also convex, and hundreds of secondary disk 
plates are mtroduced between them over the whole surface, resembling in 
form and character the primary ones in their earlier phases. Such pieces 
are found also in vast numbers near the outer margin, decreasing in size as 
they approach the arm openings. The radial dome plates —7. e. covering 
pieces — are very irregularly developed, some rays having but a single plate, 
others four or five, and while some of them are isolated, others are in con- 
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