OCULAR AND GKNITAI. IM.ATIS 



127 



occasionally in large' ones, demonstrating as elsewhere claimed that tin- incoming of oculars in 

 a matter of individual variation and not of age. There are 15 atarrants, 1%; of the*- 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. I i>ur -|M'cimen- 

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

 occur in species of the Echinidae and especially Strongylocenlrutux drolxichiensit, 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 IMflllM 

 the periproct in medium sized specimens, and two oculars in large specimen-. II. 

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

 two oculars insert is quite as frequent in small specimens as it is in larger one-. Slrngyl<>- 

 centrolus lividus is exceptionally interesting as in its dominant character and progressive variant - 

 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 variai 

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



m mm 



128 



130 



TEXT-FIGS. 128-130. Ocular plate arrangement in SlTongyloccntTotits lindu* (Lamarck). Naplo*, Italy. All 



X 4.5. 



128. Diam. 65mm. R. T. J. Coll., 697. All oculars exsert, tin- typical character. 



129. Diam. 37 mm. R. T. J. Coll., 765. Ocular I insert, a progressive variant . Compare text-fi^.. 

 129a. Diam. 24 mm. R. T. J. Coll., 698. Ocular I insert. 



130. Diam. 53 mm. R. T. J. Coll., 766. Oculars I, V insert, a progressive variant. 



Strongylocentrotus albus from the west coast of South America is an interesting ip 

 In 60 specimens, 53% have all oculars exsert like the typical character in > 



