98 SVKN LOVKN, ON THE ECHINOIDEA DESCKIBED BY LINN.EUS. 



in h 0,61, in c 0,53 in d 0,41, in e 0,54 of the diameter. The 

 basis is sliohtlv flattened, like that of A. australis Troschel. 

 and somewhat tumid at the interradia ; in the young specimens 

 c and d it is rather fiatter. The stoma is normal, in a its 

 antero-posterior diameter 0,5 7 equals that of the test. The 

 lameilar supports of the gills are rather short, the mesial nib 

 prominent. 



The calycinal system presents nothing particiilar; the sex- 

 ual pores of the adnlt are oval. 



The transverse breadth, at the ambitus, of au ambulacrum 

 is 0,30 of that of the next interradiiim, which ao-rees with 

 their mean relation in the Atlantic group. 



A striking feature in the Arbacia Lixula L. is the small- 

 ness of the spine-bearing tubercles. In a the diameter of their 

 scrobicular circle liardly exceeds two millimeters, while in the 

 A. australis Trosch. and A. asquitubcrculata Bly. it attains 

 2,5 mm., in the A. punctulata Lamck. even 3,2 mm. A peri- 

 pheral inter-radial plate measuring 12 mm. bears seven or 

 even eight tubercles, Avhile in an A. aBquituberculata of the 

 same size the correspondiug plate of 12 mm. has five or six. 

 Their vertical series are disposed in the following manner. 

 In the interradia of ö, in which I couut 17 coronal plates, in 

 h of 16, and in c and d, of 15 and 14^ tlieir peristomal be- 

 giunings being, as in all the species, concealed beneath the 

 gill-supports, they are arranged in the interradium 5 as seen 

 in the following tables, showing the lowest plate on which 

 each series begins. and the uppermost where it ends, always 

 counting from the calycinal system: 



Spec. a, diam. 45 ram., plates 17. 



Spec c, diam. 32 mm., plates 15. S]iec. d, diam. 27 mm., plates 14. 



Series a b a h 



1 begins ends 1 1 



2 > v :5 3 



3 ' 13 13 » 5 5 



4 « 12 12 « 7 7 



