130 BULLETIN OF THE 



This question can be answered in part by a knowledge of whether the 

 ring of interradials just abaxially or outside of the so-called basals (3) are 

 earlier or later in formation than the basals (3). They appear to be later, 

 for in Lud wig's Fig. 21 we have a single interradial plate just beginning 

 to form between an abaxial and an adaxial interradial (the abaxial is prob- 

 ably the oral). Moreover, this seems to be the mode of formation referred 

 to by Ludwig in the words, " Es scheiben Uberall im Bereiche des dorsalen 

 Scheibenperisoms neue Interraediarplatten zwischen und neben den ein- 

 mal gebildeten sich anlegen zu konnen." If the intermediate plates are 

 formed later, the Plate 3 in Carpenter's Fig. II. would be the same as 

 one of the ring of interradials in Ludwig's Fig. 25, which exactly corre- 

 sponds with the basals (3) of the projection of the Antedon larva. 



The determination of the relative length of the arm when the under- 

 basals and basals appear may be made by considering the relative devel- 

 opment and number of pairs of the adambulacral plates. Carpenter 

 states:* "The radials of the young Amphiura do not long remain in 

 contact with the dorsocentral ; for by the time that two adambulacral 

 plates have appeared between them and the terminals they are separated 

 from the dorsocentral by the rudimentary basal plates, while underbasals 

 are developed shortly after." In the figure quoted (PI. I. Fig. 12) by 

 Carpenter in this connection, which is probably a little older than one 

 necessary to illustrate the above statement, three pairs of side arm-plates 

 (adambulacral) are figured between the terminal {T) and the primary ra- 

 dials (4). Two pairs of lateral arm-plates at least are developed before the 

 plates which Carpenter calls basals appear. It is taken for granted that 

 the lateral arm-plates are considered adambulacral by Carpenter, — a ho- 

 mology of which there is little doubt, — and must be the ones referred to 

 by him, since they are the only plates, in the figure quoted by him (PI. I. 

 Fig. 12), between the terminals {T) and radialia (4).t 



The homology of the radial shields of the Amphiura with the first bra- 

 chials of the Crinoid would seem not unreasonable. J The only paired 

 plates of the arms with which they could be compared are the adambula- 

 cral. With these, however, they have little resemblance save in their 

 double origin. 



*D 



* On the Apical System of the Ophiurids, Quart. Journ. Micros. Sci., Vol. XXIV., 

 Jan., 1884, p. 6. 



t The first and second adambulacrals lie on the oral side of the disk, and cannot be 

 those referred to by Carpenter as appearing between the terminals and radials. 



t This suggestion is believed to be original, although it is a direct sequence of the 

 adoption of the theory that the radials of Amphiura and of the Crinoids are homolo- 

 gous, as shown above (adopted from Carpenter and others). 



