ACTINOMETRA IOWENSIS. 221 



fore described. The presence of covering plates on the ambu- 

 lacra differentiates it widely from all others of the genus. It 

 belongs to the type which Dr. P. Herbert Carpenter, in his 

 great work on the Comatulae of the Challenger Expedition, 

 called "tridistichate", — that is, having three distichals, the 

 axillary one with a syzygy, — by which he meant that it was 

 divided into two parts with a syzygial union between them. 

 Other species he called "bidistichate" which have "two dis- 

 tichals united by syzygy." (Chall. Rep. Comatufrr, p. 277). 

 This shows the confusing manner in which Carpenter employed 

 the term "syzygy", — for in these cases either the "two disti- 

 chals united by "syzygy" should be called one, or the "three 

 distichals, the axillary a "syzygy" ought to be called four. A 

 syzygial pair should be uniformly treated, either as one plate 

 or as two. The latter course is now adopted by Mr. Bather, 

 but I think there is much force in the reasoning of Carpenter 

 that the syzygial pair — at least in the arms — represents only 

 one brachial, since the hypozygal loses its individuality, and 

 bears no pinnule. The trouble is that in practice he did not 

 follow out his reasoning. Which ever plan is adopted, it ought 

 to be used consistently; and we should not call four plates, of 

 which two are united by syzygy, "tridistichate," and in the 

 same breath call two plates of the same order, also united by 

 syzygy, "bidistichate". Following the plan of Bather, the 

 form under consideration has four secundibrachs (distichals), 

 the first two united by bifascial articulation, and the next two 

 by syzygy. 



Our species belongs to the anomalous group Fimbriata, 

 comprising seven described species, which differ from all others 

 in this genus, and from all but one in Anicdon, in having 

 the first syzygy in the arms between the second and third 

 brachials; or, as Carpenter states it, "in the second brachial." 

 In all others, with the exception alluded to, the first arm syzygy 

 is either between the first and second, or the third and fourth, 

 brachials; and the first arm pinnule is borne on the second 

 brachial, instead of on the first, as in the Fimbriata group. 



