634 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, wliich 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 tliere 

 are but five pairs of arm oj^enings, which are arranged as the ten in the 

 older ones. Between these two forms there are others of intermediate stages 

 in which, althougli 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, 3, 2 ; anal inten-adius : 1, 

 3, 4, 5, 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- 

 sjjherical 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 vei'y 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 hundi-eds of secondary disk 

 plates are introduced 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 — i. 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- 



