1).  Within the genus Phalacrocidaris, adult tests of Phalacrocidaris serrifera (left) are similar to small tests of the closely related Phalacrocidaris merceyi (right).  However, the two species are arbritarily distinguished by the number of rudimentary (reduced / non-functional?) primary tubercles situated on the upper (apical) surface of each interambulacral segment.  Whereas P. serrifera has only one, tests with more than one can be assigned to P. merceyi.