312 P. HERBERT CARPENTER. 



the young Amphiura which I have described as basals (figs. 

 ii, in, 3) appear to me to be due to the fact that the sequence 

 of development of the apical plates is not the same in the 

 American as in the European variety of Amphiura squa- 

 mata. According to the description and figures of Ludwig 

 the order in the latter form is as follows : — i. Radials (4) ; 

 ii. Dorsocentral (1) ; iii. Basals (3) ; iv. Primary interradials 

 (I.), and Under-basals (2) ; and v. Radial shields (r. s.). The 

 under-basals and primary interradials seem to develop almost 

 contemporaneously, fig. in showing two under-basals (2) in 

 two rays between the dorsocentral (1) and the radials (4), and 



m&<m w 



a d^ a d a cL ^ 

 I 



Fig. III.— Apical system and one arm of the young Amphiura squamata; 

 Stage II, after Ludwig. 1. Dorsocentral. 2. Under-basals. 3. Basals. 

 4. Radials. 5. Orals. I. Primary interradials. T. Terminals. ad 3 ., ad 4 ., 

 ad & . Side arm-plates (adambulacrals). 



two very unequal abaxial or primary interradials (I.) each of 

 them between a basal (3) and what I take to be an oral (5). 1 

 This figure also shows that the fifth pair of side arm-plates 



1 It is just possible that this interpretation may be erroneous, and that the 

 plates which are marked as orals (5) in the above diagram are really the 

 primary interradials, and should be marked (I.). I do not think this probable, 

 however, as this individual is so little older than that figured by Ludwig in his 

 fig. 25, in which the orals are still quite distinct on the margin of the dorsal 

 surface. But if the plates in question be interradials appearing thus early, 

 the contrast between the times of development of the interradials and radial 

 shields in the American and European forms respectively is even greater 

 than I have described above. Eewkes agrees with me in thinking that they 

 are probably orals. 



