126 ROBERT TRACY JACKSON ON ECHINI. 



more than three elements in each ambulacral plate and other characters as discussed in the 

 section on Systematic Classification. Of this family Pseudoboletia indiana (ten specimens) 

 has in all the bivium insert. Sphaerechinus pulcherrimus (48 specimens) has 98 % with oculars 

 I, V insert and 2 % with I only insert as an arrested variant. Sphaerechinus granulans (291 

 specimens) has typically, 89 %, oculars I, V insert. Two specimens are arrested variants with 

 I only insert; three are progressive variants with I, V, IV and three with I, V, IV, II insert. 

 It is rather remarkable that this wide range of characters should be represented by so few 

 specimens, but the same limited occurrence of a typical variation occurs in many species. The 

 aberrant variants of Sphaerechinus granularis are relatively very numerous, 9%; two of these 

 have oculars V, IV insert, but the 23 others all have oculars I, V, II insert, a case of the bivium 

 and right hand instead of left hand of the trivium as usual. The frequence of this character 

 suggests that some species might typically have I, V, II insert, but such is unknown (pp. 93, 141). 



Strongylocentrotus, on account of the number of pore-pairs and plate elements in its 

 compound ambulacral plates, is the most specialized genus of the family in this important 

 structure. It belongs with this family rather than with the Echinometridae (in which it has 

 been placed) on account of the circular, not oval form, and because the oculars become insert 

 in the sequence I, V, not V, I as in that family. I have been fortunate in studying a good 

 series of most of the species and their mutual relations are very interesting (table, p. 162). 



Strongylocentrotus lividus as regards ocular arrangement is the most primitive species of 

 the genus, as it is also as regards the number of elements entering into the compound aiubulacral 

 plates. Of this species I have examined 1,163 specimens, mostly from the Naples Station. 

 Of these, as the species character, 75 % have all the oculars exsert (text-fig. 128). This includes 

 the largest specimens seen (68.5 mrh.) as well as smaller individuals. All oculars exsert is also 

 the character of very young specimens of S. drobachiensis (text-fig. 131). In lividus in 17% 

 ocular I is insert as a progressive variant (text-figs. 129, 129a). This character is like the 

 typical feature of young S. drobachiensis when 4 to 5 mm. in diameter (text-fig. 133) and also 

 like arrested variants of the same species (text-fig. 136). In lividus 2% have ocular V alone 

 insert. This is a relatively rare character in the Echinidae, but is more frequent in some 

 species of the Strongylocentrotidae, and in the Echinometridae it is the dominant character 

 when only one ocular reaches the periproct. In lividus 4 % have oculars I, V insert (text-fig. 

 130), which is a progressive variant for the species, but is the dominant character of S. dro- 

 bachiensis and most species of the genus. One specimen only, 0.1 %, has oculars I, V, IV 

 insert. This specimen has ocular IV fused with genital 3, like the ocular IV fused with geni- 

 tal 4 in text-fig. 143, p. 134. The character of oculars I, V, IV insert which is an extreme 

 and very rare progressive variant in lividus is a relatively common progressive variant and 

 frequent in an increasing degree in the higher species of the genus, as shown in the table. 

 The progressive variants in S. lividus occur in small or medium sized individuals, and only 



