108 



ROBERT TRACY JACKSON ON ECHINI. 



40 mm. or more in diameter, at which size full species characters are attained in development . 

 From several localities 1,168 specimens were studied. Of these, 57 % have oculars I, V, IV insert 

 (text-fig. 93), which is strongly the species character, and little variation from this percentage 

 was seen in specimens from any locality. The variations from the typical cover the whole 

 range of characters seen in development and also adults of all species of the family, fossil and 

 living. Two arrested variants have all oculars exsert, a feature seen in the young (text-fig. 88) 

 and in the adults of Jurassic species (text-fig. 81). Two specimens have ocular I only insert, 

 and all other oculars exsert, though V and IV are barely exsert. This character can be compared 

 with the second stage in development (text-fig. 90) and with progressive variants of Jurassic 

 species where I also is insert (text-fig. 82). These extreme arrested variants of Centrechinus 

 are small individuals (the all exsert 50 mm., the I only insert 51 mm. in diameter). One speci- 

 men has ocular V alone insert, an exceptional character. Eleven specimens, or 1 %, have as 

 arrested variants only oculars I, V insert, being comparable to the young (text-fig. 91) and to 

 an extreme progressive variant in Jurassic species (text-fig. 86). As progressive variants of 

 Centrechinus setosus, 17% have oculars I, V, IV, II insert (text-fig. 94), the bivium and pos- 

 terior pair of the trivium marking the bilateral symmetry as an order of developing parts. 



ffi 



I 

 96 "' 98 



TEXT-FIGS. 96-98. Ocular plate arrangement in the Centrcchinidae. 



96. Centrostephanus rodgersi A. Agassiz. Parramalta, N. S. W. Diam. 110mm. R. T. J. Coll., 750. X 2. Oculars 

 I, V, IV insert, madreporite split, young ambulacral plates simple (p. 174). 



97. Centrostephanus longispinus (Philippi). Naples Station. Diam. 27 mm. R. T. J. Coll., 751. X 2. All oculars 

 insert, plates of periproct numerous and minute, excepting a few large plates on the periphery of the area (p. 174). 



98. Chaetodiadema pallidum A. Agassiz and Clark. Hawaiian Islands. Diam. 65 mm. R. T. J. Coll., 700. X 2. 

 All oculars widely insert, plates of periproct large (p. 174). 



In 22% all the oculars are insert (text-fig. 95). This character, which is a progressive variant 

 of Centrechinus setosus, is the typical feature of Echinothrix, Centrostephanus longispinus (text- 



