OCULAR AND GENITAL PLATES. 



127 



occasionally in large ones, demonstrating as elsewhere claimed that the incoming of oculars is 

 a matter of individual variation and not of age. There are 15 aberrants, 1%; of these one 

 specimen has ocular III alone insert, a very rare variant in Echini, seen in only two other 

 specimens. Five specimens have ocular II alone insert, also a rare variant. Four specimens 

 have I, IV insert, two have I, II, and three have V, II insert. These are aberrants that 

 occur in species of the Echinidae and especially Slrongylocentrotus drobachiensis, as shown 

 in the table of aberrants. A large number of specimens of lividus were received after the 

 diagram (text-fig. 176) was drawn and these altered the percentages considerably from 

 what is there shown. Mr. Agassiz (1873, p. 446) said of this species, that one ocular reaches 

 the periproct in medium sized specimens, and two oculars in large specimens. He 

 did not mention all oculars exsert, which in those I have seen is the dominant character; also 

 two oculars insert is quite as frequent in small specimens as it is in larger ones. Slrongylo- 

 centrotus lividus is exceptionally interesting as in its dominant character and progressive variants 

 it presents features in arrangement of oculars precisely like those seen in the development of 

 S. drobachiensis (text-figs. 131-134); also its typical character and progressive variants are 

 directly comparable to the arrested variants or typical character and progressive variants of 

 adult drobachiensis (text-figs. 135-138, p. 132). 



n nr 



Text-figs. 12S-1.30. — Ocular plalo arrangoincnt in Slmiigyloccnlrolvs lii'idiis (Lamarck). Naples, Ilaly. All figures 

 X 4.5. 



128. Diani. (i.'j mm. R. T. .J. Coll., 097. All oculars exsort, the typical character. Compare Icxt-figs. 132, 13.5. 



129. Diam. 37 mm. R. T. J. Coll., 705. Ocular I insert, a progressive variant. Compare text-figs. 133, 136. 

 129a. Diam. 24 mm. R. T. J. Coll., 09S. Ocular 1 insert. 



130. Diam. 53 mm. II. T. J. Coll., 700. Oculars I, V insert, a progressive variant. Compare text-figs. 134, 137. 



Slrongylocentrotus albus from the west coast of South America is an interesting species. 

 In 60 specimens, 53 % have all oculars exsert like the typical character in ,S'. lividus. A con- 



