THE CRINOIDEA 115 



with the next Br, representing the IIIBr of a simple arm. To use 

 the same terms is especially perplexing in intermediate forms ; 

 but solutions of the difficulty, though proposed, have not gained 

 general approval. 



For .descriptive purposes, a dichotomous arm is viewed from 

 the dorsal, i.e. aboral surface, and " right " and " left " equal right 

 and left of the observer. The mediad rami are called interior ; 

 those to the sides, exterior. A particular brachial in any series 

 is denoted by placing a small Arabic numeral after the symbol 

 IIBr 4 , IVBr (; , IIIBr.. The number of Br in a series may be 

 expressed either by stating it with a large Arabic numeral, e.g. 

 IIIBr, 7, and IIBr,10; or by giving the number of the axillare, 

 thus Illax-, and IIax 10 . The ossicles of the distal rami which do 

 not branch again are called " finials " (F). 



The ambulacrals (Amb) 1 necessarily branch with the brachials, 

 and the several series or orders may be designated as lAmb, 

 IIAmb, etc. Their simplest form is that of a line of small 

 plates on either side the groove, capable of being raised or 

 depressed ; and when closed, meeting in the median line by a 

 zigzag suture due to their alternating arrangement. They vary 

 in size, both absolutely, and relatively to the brachials. Each 

 ambulacral may be divided by one or more tranverse sutures, 

 parallel to the long axis of the arm ; this produces the appearance 

 of short pinnules, for which these structures have been mistaken 

 by more than one author. The transverse sutures may come to 

 lie at an angle, and the portions to alternate with one another. 

 Thus arose the side-plates or adambulacrals, which are a persistent 

 feature in many of the later crinoids (cf. Fig. IX. 2). There 

 may also be developed minute but distinct ossicles beneath the 

 outer covering-plates and alternating with them. The complicated 

 structure thus developed in Cyathocrinus and Gissocrinus has been 

 exquisitely worked out and illustrated by G. Liljevall (Bather, 

 1893, pis. vii.-ix.). 



Brachia, rami, ramuli, and pinnulae, in which the ossicles lie in 

 a single row, with more or less parallel joints, are termed " uni- 

 serial." Simple arms in Crinoidea are always uniserial. Pinnu- 

 late arms undergo a modification. Since in such arms the joints 

 slope alternately to right and left (p. 114), the brachials tend to 

 assume a wedge-shape; in process of growth of either the in- 

 dividual or the race, a complete wedge-shape is assumed, so that 

 the joint-lines between the ossicles form a " zigzag." Lastly, the 

 brachials come to lie in two alternating rows, in which case the arm 

 is termed "biserial." This development doubles the number of 

 pinnules in a given length of arm, and thus aids the collection 



1 The terms "ambulacral" and " adambulacral " must not be held to imply any 

 homology with elements thus named in Stelleroidea and Echinoidea. 



