224 REPORT OF NATIONAL MUSEUM, 1901. 



The brachia are rarely single, usually bifurcating in a regular or irregular manner 

 at definite points. The pinnule (colored black) is a brachium in miniature; itdiffers 

 in nothing but position from the small end branches of a simple dichotomous arm. 

 However, when the pinnules are regularly placed on alternate sides of successive 

 brachials of the main branch the arm is said to be pinnulate. To these pinnules are 

 restricted the fertile portions of the genital rachis. 



24,193. Scytalocrinus robustus. Lower Carbonic. Brachia pinnulate with 

 regular bifurcations. Brachials in single columns. 



34,087. Platycrinus agassizi. Lower Carbonic. Brachia pinnulate with regu- 

 lar bifurcations. Brachials in double, alternating columns. 



24.191. Cyathocrinus multibrachiatus. Lower Carbonic. Brachia dichoto- 



mous, without pinnules. 

 24,155. Barycrinus hercules. Lower Carbonic. Pinnulate brachia showing 

 the ambulacral groove and ambulacral plates. 



THE ANAL AREA. 



(See Plate 66.) 



In most crinoids there is inserted in an interradius one or more extra pieces called 

 anal plates (colored red). The anal tube when present (colored blue) is supported 

 by the anal plates. The anal opening ( colored brown) may be in the side or at the 

 apex of the tube. 



26,771. Cactocrinus ornatissimus. Lower Carbonic. Showing the anal tube 

 between the brachia. 



24.192. Scaphiocrinus unicus. Lower Carbonic. Showing the anal tube with 



the anal opening in the side. 



24.193. Scytalocrinus robustus. Lower Carbonic. 



27,868. JEsiocrinus magnificus. Upper Carbonic. The brachia have been 

 removed. 



ENLARGEMENT OF THE SIMPLE DORSAL CUP. 



(See Plate 63*. ) 



The dorsal cup may be enlarged by the addition of proximal brachials (here col- 

 ored blue) or by the insertion of a variable number of extra plates between the 

 brachials. The latter are called interbrachials (colored red). 



24,156. Taxocrinus multibrachiatus. Lower Carbonic. 

 24,163. Batocrinus wachsmuthi. Lower Carbonic. 



THE STEM. 



(See Plate 67. ) 



The flexible stem is composed of stem ossicles, which vary not only greatly in num- 

 ber in different crinoids, but often also in form in different parts of the same stem. It 

 has a more or less large axial canal which serves to transmit vascular and nervous 

 pr< ilongations, and in the earlier forms may have served other purposes. The larger 

 segments are called nodal ossicles (blue), and may bear cirri (red) at regular or irreg- 

 ular intervals throughout the whole length of the stem. 



In some forms the stem is almost absent, the cup then cementing to solid objects. 

 In Pentacrinus the stem attains a length of 18 feet. In rare cases the stem has no 

 roots, but attaches by the cirri or by windingthe stem around the object of attach- 

 ment. In others it ends in a four-fluked grapnel or in a bulb; finally, there are 



