BEVISION OF PALEOZOIC STELLEROIDEA. 259 



the evidence at our disposal. But it is doubtful whether it can 

 be disproved, and I therefore make it to promote a search for 

 further evidence pro or con. 



"Two other features in the present specimen need discussion, 

 and may perhaps throw light on the problem just posed. The 

 first of these is the fact that the broadest and stoutest portion of 

 each arm segment appears to be the distal half (marked a in fig. 1), 

 whereas in the developing Ampldura and in the adult Protasteridse 

 the reverse is the case. This character is also found in Furcaster 

 palseozoicus as figured by Mr. Stiirtz, in Ptilonaster princeps of 

 J. Hall, and in Lapworihura M'dtoni, as interpreted by Dr. Gregory. 

 (See our fig. 4 [30].) In the Epphaven fossil the feature is to some 

 extent accounted for by the supposed fusion of a spiniferous adam- 

 bulacral element with the distal wing of the ambulacral; but in 

 Lapworthura and Furcaster this wing, while unconnected with the 

 adambulacral, is still more preponderant. In some other early 

 Palaeozoic genera, e. g. Eugaster, J. HaU, and Eopliiura, Jaekel, 

 the -xnng is neither distal nor proximal, but arises half way up the 

 ambulacral, so that one can not say to which of two successive seg- 

 ments the podium belongs; in fact, the structure is in this respect 

 absolutely that of an Asterid.^ These difl'erences call for some 

 explanation. Starting from the indifferent Ordovician type of 

 Eophiura, we may suppose that in one series of genera (e. g. Protas- 

 teridse) the wing moved toward the mouth, while in another series 

 (e. g. Lapworthurida?) it moved away from the mouth. In both 

 series the podium ultimatel}^ became inclosed withm the substance 

 of (apparently) a single ossicle; but, if this took place by a sub- 

 sequent lateral outgrowth of stereom, then in the former case the 

 vertebra must have inclosed a podium distal to itself, and in the 

 latter case a podium proximal to itself. Thus the relations of the 

 vertebrae to the podia would be fundamentally different in the 

 two lines of descent. On the other hand, alternate podia may, 

 as suggested on a previous page, have been inclosed by the imion 

 of successive pairs of vertebrae, and this type of structure may 

 be the one seen in process of development in the arm segments of 

 our fossil, and the one that persisted to the present day. 



"The second feature worthy of attention is the considerable 

 space that seems to intervene between the distal wing of one seg- 

 ment and the proximal wing of the next. Tliis is conspicuous in the 

 distal region of ray i, where, further, the depression (q in fig. 1 [32 B]) 

 markedly resembles the podial depression (p in fig. 1), and differs 

 from "it mainly in being rather nearer the axis of the arm. The 

 theory that the Opliiurids are descended from Asterids does not 



1 Sec Jaekel, Zcits. geol. Oes., vol. oo, Protok., 1904, pp. 10r.-n3. 



