234 HAWAIIAN AND OTHER PACIFIC ECHINI. 



an elongated test is associated with only three pairs of pores in each ambulacral 

 plate. As a further indication of the intermediate position of Parasalenia, 

 attention may be called to the fact that the elongation of the test in that genus 

 is through IIIb-5b, while in Echinometra it is ordinarily through 3-1 and in 

 Heterocentrotus, the most extreme of the family, it is through IVb-lb or 4a-IIa. 

 There is thus a progressive movement of the axis to the animal's left, which is 

 least marked in Parasalenia. It seems that it would be perfectly proper to 

 class that genus with the Echinidse if one preferred, although for convenience 

 it is here placed in the Echinometridse. 



(4) The genus Echinostrephus, in a somewhat similar manner, serves to 

 connect the Echinidse, characterized by only three pairs of pores in each 

 ambulacral plate, with the Strongylocentrotidse, characterized by more than 

 three pairs. For in one species of Echinostrephus there are three pairs of pores 

 and in the other there are four. The line of division between the two families 

 thus cuts the genus Echinostrephus exactly in two and it might therefore be 

 properly placed in either family. On account of the specialized form of the test 

 however, I place it in the Strongylocentrotidse, and consider this view strength- 

 ened by the fact that specimens of Strongylocentrotus, of two different 

 species, occur in which the ambitus is above the mid-zone, as is so character- 

 istic of Echinostrephus. This peculiarity has not been noted in any species of 

 the Echinidse. 



Since so much stress is laid on the importance of the number of pore-pairs in 

 an arc, or rather on the number of elements in each ambulacral plate, it is proper 

 to utter a word of caution in regard to the use of this character. It must con- 

 tinually be borne in mind that the oldest plates in each ambulacrum are those 

 at the peristome, while the youngest are next to the ocular plate. The former 

 often show therefore features characteristic of recent ancestors, while the latter 

 show youthful characters, i. e., are not fully developed. Consequently it is in 

 the area between these two regions, aptly termed by Jackson the "mid-zone," 

 that the specific characters must be sought. For practical purposes, the region 

 just above the ambitus (in Echinostrephus, just below the ambitus) will show the 

 species character in adult specimens. In young specimens, obviously the species 

 character will be further above the ambitus and in very young specimens it will 

 be found, if at all, near to the ocular plate. On the other hand in very old 

 specimens, old age characters will begin to appear near the ocular, senescence 

 having set in. In an old specimen therefore of a Strongylocentrotus with six 

 elements in the ambulacral plates, as its specific character, there will be found 



