260 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



explain the evolution of such an interspace, and the general tendency 

 to shortening and approximation of the vertebrae, shown in the 

 history of the Ophiurids, leads one to inquire why there should 

 ever have been introduced a stage in which, as in the Devonian 

 OpJiiurina, the recent deep-sea Ophwhelus, and the young AmpTiiura, 

 the vertebrae have so far departed from the primitive type as to 

 become axially elongate. This stage has been regarded as primi- 

 tive by Lyman, Ludwig, and other zoologists, but in theory and 

 fact it is ' primitive' only so far as the later forms are concerned, 

 while with reference to the complete series it is 'intercalated.' 



"Recurring to the hypothesis that the vertebrae are compounded 

 of successive pairs of ambulacrals, we find in it an explanation 

 not merely of this curious elongation of the arm segments in general, 

 but of the interspaces between them in this genus (lettered q). An 

 objection to the hypothesis was the necessity for assuming the sup- 

 pression of alternate podia. Here, it may be, is evidence of a 

 stage in which those podia were not yet suppressed, though perhaps 

 somewhat atrophied. If both depressions (p and q) were for the 

 reception of podia, then their alternate approximation to and re- 

 moval from the perradius may be compared with the similar phe- 

 nomenon in Asterids. 



" Whether the features just discussed be or be not accepted as 

 evidence in favor of the compound origin of the Ophiurid vertebra, 

 they demand some explanation; and it may be added that the same 

 hypothesis will perhaps furnish an equally needed explanation for 

 the hitherto unexplained differences that obtain between Palaeo- 

 zoic genera of Ophiurids in the relations of the adambulacrals to the 

 ambulacrals. 



"The origin of the Ophiurid mouth-skeleton also may be illu- 

 minated by the preceding hypothesis, which is in full harmony with 

 the opinion that many more arm segments enter into the composition 

 of that structure than is the case in Asterids, an opinion based on 

 embryological research by Dr. O. zur Strassen, 1 and on palaeonto- 

 logical investigation by Dr. O. Jaekel in the paper quoted above. 

 In this region of the ray, at any rate, adambulacrals and podia must 

 have been suppressed in all Ophiurids. We may here note the 

 apparent absence from our specimen of the adambulacral elements 

 known as side mouth-shields, as well as of the peristomial plates. 

 The latter are wanting in most early Ophiurids, a fact confirmatory 

 of Dr. zur Strassen's conclusion that they are not ambulacral elements, 

 but ' secondary calcifications confined to the interradial region.' ' 



Schondorf states that the structure of the arms is like that in 

 LapwortJiura miltoni and the oral armature as in Encrinaster. 



i Zool. Anzeiger, vol. 24, 1901, p. 609. 



