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plates, developed on the left antimere and connected with the mouth, 

 viz., orals and all plates of the ambulacra. The secondary or supple- 

 mentary elements are all the interradial, interbrachial, and inter- 

 ambulacral plates, including the anal plates and those of the tube or 

 sac. It is, apparently, to these secondary plates that Wachsmuth and 

 Springer restrict the term perisomic. 



It is, then, perfectly clear that Dr. Lang's definition is something 

 quite new ; and he ought, therefore, to have said as much, and to 

 have given the grounds on which it was based. What those grounds 

 may be I am at a loss to discover : to include brachials and inter- 

 brachials under one head seems a hopeless confusion. However 

 advisable it may be to discriminate between the supposed primitive 

 apical and oral systems on the one hand, and the supposed newer 

 additions on the other, it must always remain unjustifiable to wrest 

 the word perisomatic from its original and usual meaning. It seemed 

 possible that Dr. Lang might have been influenced by the use of the 

 word in other orders of the Echinoderma ; but it does not appear 

 that even there the word is used (or, if used, properly used) in the 

 sense here attached to it. Huxley, in "A Manual of the Anatomy of 

 Invertebrated Animals," 1877, uses " perisoma " in its obvious sense 

 for the general body wall of Echinoderma ; but he does not use the 

 words " perisomic " or " perisomatic," and does not differentiate the 

 skeletal elements. In its simplest meaning, derived from the fore- 

 going simple meaning of " perisoma," the word " perisomatic " would 

 naturally be applied to the skeleton of the body-wall as opposed to 

 that occasionally deposited in the viscera; but this would bring us no 

 nearer Dr. Lang's use of the term. In no other of the standard text- 

 books is the word used in any such sense, and it, therefore, seems right 

 to raise an immediate protest. For my own part, I should prefer to 

 drop the term altogether. 



It was hardly to be expected that the difficulties that have been 

 introduced into the study of crinoid arms by many generations of 

 writers should have been entirely mastered by Dr. Lang. He is 

 correct in discriminating the primary brachials, which are purely arm 

 structures, from the radials, which belong to the apical system. He 

 also points out very clearly how certain brachials may become 

 included, as fixed brachials, in the limits of the dorsal cup, and yet 

 remain morphologically part of the arms. He does not, however, 

 bring out with similar clearness the fact that the number of arms in 

 every crinoid, with a few well-known exceptions, is five, and that any 

 apparently greater number is produced solely by the branching of 

 these five arms. Had this been clear to the writer's mind, he would 

 not have wasted space in repeating the fact that there are five arms 

 in Thawnatocrinus, five in the Bourgueticrinidae, and so on ; nor would 

 he have spoken of " ten pairs of arms " in the Eucalyptocrinidae when 

 he meant ten pairs of arm-branches; nor would he have said "arms 

 usually branched " in the Apiocrinidae, since they are, if I remember 

 rightly, always branched, though sometimes only once. 



There is a similar hesitancy in the various references to pinnules. 

 It is suggested, with undoubted correctness, that " the pinnules are 

 best regarded as the ultimate arm-branches " ; but had the author 

 really understood what this sentence implied, he could not have 

 written, "pinnules appear to be absent" in Crotalocrinus. It is no 

 question of appearance : pinnules simply could not be present, any 

 more than the most angelic of us could sprout wings. 



This confusion extends to the description of the covering-plates 

 of the arms. These are minute plates developed on either side of 



