166 Dr. F. A. Bather— A New Ophiuricl 



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

 •palceozoiciis as figured by Mr. Stxirtz, in Ptilonaster princeps of 

 J. Hall, and in Lapworthura Miltoni as interpreted by Dr. Gregory 

 (see our Fig. 4). In the Epphaven fossil the feature is to some 

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

 lacral 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. Hall, and Eopliiura, Jaekel, the 

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

 ambulacral, so that one cannot say to which of two successive 

 segments the podium belongs : in fact, the structure is in this 

 respect absolutely that of an Asterid.^ These differences call for 

 some explanation. Starting from the indifferent Ordovician type 

 of Eopliiura, we may suppose that in one series of genera (e.g. 

 Protasteridee) the wing moved towards the mouth, while in another 

 series (e.g. Lapworthurid?e) it moved away from the mouth. In 

 both series the podium ultimately became enclosed within the 

 substance of (apparently) a single ossicle ; but, if this took place 

 by a subsequent lateral outgrowth of stereom, then in the former 

 case the vertebra must have enclosed a podium distal to itself, and 

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

 of the vertebrse 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 enclosed by the union 

 of successive pairs of vertebras, and this type of structure may be 

 the one seen in process of development in the ai'm-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 segment and 

 the proximal wing of the next. This is conspicuous in the distal 

 region of ray i, where, further, the depression {q in Fig. 1) 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 Ophiurids are descended from Asterids does not explain the 

 evolution of such an interspace, and the general tendency to 

 shortening and approximation of the vertebrfe, shown in the history 

 of the Ophiurids, leads one to enquire why there should ever have 

 been introduced a stage in which, as in the Devonian Ophiurina, 

 the recent deep-sea Ophiohelus, and the young Amphiura, the 

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

 axially elongate. This stage has been regarded as primitive 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 



' See Dr. 0. Jaekel's interesting account of genera from the Lower Palin'ozoic 

 rocks of Bohemia, intermediate between Asterids and Ophiurids : Zeitschr-. Deutsch. 

 geol. Ges., Iv, Protok. pp. 106-113; 1904. Eeferences to the older literature wUl 

 he found in Dr. Gregory's oft-quoted paper. 



