OCULAR AND GENITAL PLATES. 125 



ing and a developed series. In the developing series of 55 specimens, varying from 19 to 50 

 mm. in diameter, 76 % have the bivium insert, quite near to the character of adults from Ber- 

 muda. In 9% ocular I only is insert. Of progressive variants, 13% have oculars I, V, IV 

 insert. This is much below the percentage of adults, in which I, V, IV insert is the character 

 in this region. One aberrant has oculars I, V, II insert. 



In the developed series of 50 to 127 mm. diameter from the West Indies and Florida (455 

 specimens), in 38% oculars I, V, IV are insert as the typical character (text-fig. 125). This 

 is the only species of the family in which I, V, IV has been found as a dominant character, 

 although it is a frequent variant in many species. As arrested variants two specimens, large 

 adults, have V only insert. In 36% oculars I, V are insert (text-fig. 124). This is an arrested 

 variant for the southern, while it is the typical character for the Bermuda foi-m of the species. 

 As progressive variants 18% have oculars I, V, IV, II insert (text-fig. 126). This variant is 

 important as it includes the bivium and posterior pair of the trivium, thus marking the bilateral 

 symmetry in regular Echini as indicated by the incoming of ocular plates. In 1 % all the 

 oculars are insert (text-fig. 127). The aberrant variants of the southern form of Tripneustes 

 esculentus are rather frequent, 6%. Of the 27 aberrants, one has oculars I, IV insert, as in 

 text-fig. 144, and 24 have oculars I, V, II insert, which is the bivium and right posterior plate of 

 the trivium instead of the left as usual. One specimen has V, IV, II insert, in which ocular I 

 is excluded by the fusion of genitals 5, 1 (text-fig. 196) ; and one has I, V, IV, III insert, a rare 

 variant in the order but common in the Cidaroida. The difference in Tripneustes esculentus from 

 Bermuda and the West Indian area appears to be marked only in the arrangement of the ocular 

 plates, no other distinction being observed. It is the same difference noted in several other 

 species where series from widely separated areas are tabulated. It shows differential characters 

 developing, not distinguishable in a single specimen, but obvious in a large series. Such may be 

 considered as incipient species in the process of making. In the total number of 703 specimens 

 tabulated, it is interesting to note that of 30 aberrants, 26 are cases of I, V, II insert, and of the 

 other four cases in three (p. 164) the aberration was due to the fusion of two genitals 

 mechanically shutting out an ocular. 



Of Evechinus chloroticus (20 specimens) in 90 % oculars I, V are insert, one specimen is a 

 progressive variant with I, V, IV insert, and one specimen is aberrant with I, IV insert. 



Considering the family Echinidae as a whole, many species of the genus Echinus have all 

 oculars exsert as a character. One species. Echinus magellanicus, has ocular I only insert, and 

 many species in the family have typically oculars I, V insert. Onl,y one species, Tripneustes 

 esculentus, has typically oculars I, V, IV insert and this only in its southern localities. Four 

 or five plates insert are rare in the family except in the southern foini of Tripneustes esculentus 

 and the Antarctic species of Echinus (table, pp. 160, 161). 



The Strongylocentrotidae includes genera in which the test is circular in outline, with 



