420 prof. f. j. bell, on the echinometrid^. [mar. 15, 



Heterocentrotus mammillatus. 



Heterocentrotus trigonarius. 



As compared with //. mammilluixis, we may note tlie striking 

 equality of the lengths of the true and of the apparent long axes of 

 the actinostome in this species. 



Colobocentrotus. 



The proportional measurements of the two species of this genus 

 exhibit the greatest variability ; and here, as in the case of Hetero- 

 centrotus, there is no marked diminution in the proportional value 

 of the diameters of the actinal and abactinal series as the test 

 increases in size. 



This striking deviation from the ordinary rule does not show itself 

 when the genus Echinometra proper comes to be studied ; and it is 

 impossible to resist the suspicion that the " obliquity " of the long 

 axes of Heterocentrotus and Colobocentrotus on the one hand, and 

 Echinometra on the other, is not altogether to be referred to the 

 same cause. 



^ The percentage yalues are, of course, calculated from the morphological 



axis. 



