146 ROBERT TRACY JACKSON ON ECHINI. 



155), but yet retains the specific irregularity. Eight have V, IV, III insert (text-fig. 156), 

 a very unusual arrangement, seen in only three other cases in the order Centrechinoida, yet 

 it is the dominant character, 33 %, in S. gibbosus. This arrangement of oculars with V, IV, 

 III insert was found as an aberrant variation in six cases in the Cidaroida, as described 

 in Cidaris affinis and Eucidaris tribuloides (pp. 96, 97, 99). Six specimens, 25%, have oculars 

 I, V, IV, III insert, again a rare arrangement in the order, but frequent in the Cidaroida. 

 In young specimens of gibbosus a few mm. in diameter all oculars are exsert. In the Strongy- 

 locentrotidae, of the species studied, two are characterized by having oculars all exsert; all 

 others have typically the bivium insert iexcept the aberrant S. gibbosus (pp. 162, 164). 



The Echinometridae as here restricted includes Echinometra and allies, which are elliptical 

 in outline through a sidewise axis, a character peculiar to this group of Echini only. In this 

 family, as in the Arbaciidae, when ocular plates enter the periproct, it is in the sequence V, I, 

 not I, V, as ill the Echinidae and Strongylocentrotidae. On this basis, when only one ocular 

 is insert, it should be V, and this is true as a dominant rule; occasionally, however, ocular I 

 alone is insert. Such cases may be considered variants toward the character of the associated 

 families in which, if only one plate is insert, that plate is typically ocular I. The rule is not 

 quite so constant as in the Arbaciidae, in which, if one plate is insert, it is ocular V with very 

 rare exceptions. The most primitive species of Echinometra on the basis of ocular plate 

 arrangement is E. oblonga. Of this species (44 specimens) in 98 % all the oculars are exsert, 

 and in 2 % ocular V is insert as a progressive variant. In Echinometra viridis (25 specimens), 

 96% have all oculars exsert, and 4% ocular V insert. Echinometra mathaei {15Q specimens) 

 has typically, 79 %, all oculars exsert; of progressive variants 19 % have V, and 1 % have V, 1 

 insert; 1 % is aberrant with oculars V, II insert. 



Of Echinometra lucunter a large series was studied, and it shows different facies in two 

 widely separated localities. In the West Indian area (578 specimens) typically, 57 %, ocular 

 V alone is insert (text-fig. 159). As arrested variants 17 % have all oculars exsert (text-fig. 

 158), the typical character of E. oblonga. As progressive variants 25 % have oculars V, I 

 insert (text-fig. 160), like the typical character of E. van brunti, and 0.3 % have V, I, IV insert 

 (text-fig. 161). In Echinometra lucunter from Bermuda considerable difference exists. In 

 176 specimens from that locality ocular V is typically insert in 46 %. The arrested variants, 

 however, are less frequent, only 9 % having all oculars exsert. Also the progressive variants 

 are much more frequent than in the West Indian form, there being 43 % with V, I, and 0.6 % 

 with V, I, IV insert. The Bermudan form, while having typically ocular V insert as in the 

 West Indian, is therefore distinctly more progressive as shown by its variants, and makes a 

 closer approach to the character seen in the next higher species, E. van brunti. The Bermudan 

 form of E. lucunter is notable for its large size and robust character. One specimen in my 

 collection from that locality measures 90 mm. through the long axis. Wliile in Echinometra 



