NOTES ON ECHINODEEM MORPHOLOGY. 315 



me considerably, as he has omitted to letter the two plates 

 next to the radial primary of the ray which he figures. I 

 have marked them (4), regarding them as the primaries of the 

 adjoining rays. But from his remarks on p. 128, which I 

 have just quoted, it seems possible that they may be his adaxial 

 interradials, i.e. the true basals, which appear before the 

 radial shields, and not after them, as I have — perhaps erro- 

 neously — supposed. On the other hand, he states on the 

 same page that a third ring of interradials " arises between 

 the last- formed interradials and the periphery of the dorso- 

 central." Which does he consider as representing the Cri- 

 noidal basals ? The second, or the third ring which is the 

 most adaxial ? There is not a single reference to his figures 

 in the whole course of his description on p. 128 of the develop- 

 ment of the plates in the interradial areas, and I have there- 

 fore found much difficulty in understanding it. 1 say this 

 because I wish to apologise in advance if I have misinter- 

 preted his statements. 



The under-basals, which are formed comparatively early in 

 the European Amphiura squamata, do not seem to appear 

 in the American variety till the radial shields have reached 

 some size. If we take the number of adambulacral plates as 

 a criterion, we find that the European form with three of these 

 plates (fig. m), has five basals, two under-basals, and two inter- 

 radials, 1 but no radial shields ; while the American form at the 

 same stage has small shields and large interradials, but neither 

 basals (?) nor under-basals (fig. iv). The differences in the 

 sequence of formation of the apical plates in the two types 

 may be summarised as follows : 



European : American : 



Radials (4). Radials (4). 



Dorsocentral (1). Dorsocentral (1). 



Basals (adaxial interradials, 3). Abaxial interradials (I.). 



( Abaxial interradials (I.). 2 Radial shields (;•. s.). 



t Under-basals (2). 2 Basals (adaxial interradials, 3). 



Radial shields (r. s.). Under-basals (2). 



1 See note 1 on p. 312. 



2 It is possible that these two should be interchanged. 



