No. 515] AFFINITIES OF THE EC II IX OWE A 



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crease by the addition of plates proximally 

 the youngest plate and the radial (ocular) ; no permanent 

 terminal plate is present; (2) the ambulacral plates al- 

 ways alternate in position; (3) the so-called infrabasals 

 are inconstant (crinoids) or entirely wanting (ecliinoids 

 and many crinoids); (4) imbrication of ambulacrals is 

 more or less constant; (5) the ambulacrals are on the 

 surface, so that the nerves, water vascular system, 

 schizoccel, etc., are on the inner side, protected by them; 

 (6) there are no definite or constant accessory plates in 

 the ambulacral system; (7) the oral skeleton, when pres- 

 ent, can not be directly derived from the ambulacral 

 system; (8) the interambulacral plates are in more or 

 less regular columns ; but they always start from a single 

 plate; (9) the interambulacral plates extend laterally out- 

 ward from the ambulacrals, forming a closed capsule; 

 (10) the plates of the ambulacral system encroach regu- 

 larly, when at all, upon the peristome, which remains 

 round; (11) the spines are long and usually slender, and 

 are attached to round and prominent spine bosses (pres- 

 ent among crinoids in one genus only) ; (12) the ambula- 

 crals are directly continuous with, and in the same plane 

 as, the radials (oculars) : on the other hand, in the ophiu- 

 roids and asteroids (1) the ambulacrals increase by the 

 addition of plates distally between the youngest plates 

 and the permanent terminal; (2) the ambulacrals are 

 always opposite each other (a possible exception in cer- 

 tain paLToasteroids) ; (3) the infrabasals are ontogenet- 

 ically constant; (4) there is no imbrication of the ambula- 

 crals; (5) the ambulacral ossicles lie deep, so that the 

 radial structures are outside of them; (6) definite and 

 constant adambulacral ossicles or lateral shields are 

 present; (7) there is a peculiar oral skeleton of modified 

 ambulacral plates ; (8) the interambulacral plates are in 

 regular rows, starting as some multiple of two; (9) the 

 interambulacral plates enclose more or less the ambu- 

 lacral plates; (10) the plates of the ambulacral system 

 encroach irregularly upon the peristome, making it more 

 or less sharply stellate; (11) the ambulacral plates 



