18 



BULLETIN "li, UNITED STATES NATIONAL MUSEUM. 



Tabic of meastirements. 



[Note.— ac= the plates of the actlnal side; ab= those of the abai-linal side. 



Arxosoma belli. 



Arxosoma fenestralum. 



Arseosoma violaceum. 



Calveria hystrix. 



a ,\bove the ambitus. 

 b Without genital openings. 



cToo badly preserved for counting the abactinal plates. The present specimen is not the one figured on Plate 13, fig. 1. 

 Sec explanation of this plate. 



The measurements and enumerations given here show clearly that, as regards 

 the relative dimensions of the different parts of the test, there are no very reliable 

 diiferences to be found between ^1. belli and the other three species from the 

 Atlantic with which it must be compared, namely, A. f cue stratum, A. violaceum, 

 and Calveria hystrix. The last upon the whole appears to have a somewhat larger 

 number of ambulacral and interambulacral plates, and A. violaceum likewise seems 

 to have a proi^ortionatcly larger number of coronal plates than ^1. belli. 



Regarding the siiape of the plates there are evidently some rather important 

 differences. Professor Bell, it is true, maintains" that these differences are quite 

 unreliable, all transitional forms occurring. Since, however, he does not hold A. 

 fenestratum as sj)ecifically different from ( '. hystrix (of which there is not the slight- 



"■ On the Echinoderms collected by the S. S. Fingal in 1890 and by the S. S. Harlequin in 1891 off 

 the west coast of Ireland. Proc. Hoy. Dublin Soc. (n. s.), vol. 7, 1892, p. 528, pi. 25. 



