490 



NORTH AMERICAN INDEX FOSSILS. 



are called brachials. The first brachials following the R are called 

 costals or brachials of the first order (primibrachs) ; there are 

 usually one or two in vertical series, rarely more (Fig. 1804, 

 r\, C-, c^). When in following up the series of costals (primi- 

 brachs) from the R, one is 



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Fig. 1804. Analysis of the calyx of a camer- 

 ate crinoid. ib, infrabasals ; B, basals or 

 parabasals ; Bp, posterior basal ; R, radials; 

 IR, interrradials ; IRA^ anal interradius ; c^, 

 c^, r\ first, second and third costals or primi- 

 brachs. 



found that bears on its up- 

 per face two plates instead 

 of one, or one plate and a 

 pinnule, thus giving rise 

 to two succeeding series, 

 the plates of these two suc- 

 ceeding series are called 

 distichals or brachials of 

 the second order (secundi- 

 brachs). In like manner 

 distichals give rise to pal- 

 mars or brachials of the 

 third order (tertibrachs). 

 Succeeding divisions, the 

 postpalmars, are not named 

 but when referred to are 

 called brachials of the fourth, fifth, etc., orders {quartibrachs, 

 etc.). The bifurcating plates which give rise to these successive 

 orders are called axillaries. These plates may form part of calyx. 

 The free arms may be simple or branching. When small lateral 

 appendages are given off alternately from opposite sides of the 

 arms, these are called pinnules. The arms are uniserial when their 

 joints extend through to both sides of the arms (Fig. 1806, b) ; 

 biserial when they interlock from opposite sides. Biserial arms are 

 uniserial in their lower parts, these passing through wedge-shaped 

 stages to the biserial condition. The apical portion of biserial arms 

 is also uniserial (see Fig. 1806). When two or more arm plates 

 are united transversely by a rigid suture, and only the upper plate 

 bears pinules, they are said to form a syzygy; the lower non- 

 pinulate plate is called the hypozygal, and the upper the epizygal, 

 the two constituting one element. The rays and their subdivisions 

 may be laterally in contact; they are, however, usually separated 

 by supplementary plates. Such supplementary plates receive 



