106 



ROBERT TRACY JACKSON ON ECHINI. 



I is nearly in. In text-fig. 90 ocular I is insert, the others all exsert, a developing condition 

 like the progressive variant of Jurassic Pseudodiadema (text-fig. 82). A later stage is shown 

 in text-fig. 91, in which oculars I and V are insert and IV is nearly in. This may be compared 

 with the extreme progressive variant in Jurassic Stomechinus (text-fig. 86), in which also the 

 bivium is insert. A still later stage (text-fig. 92) has oculars I, V, IV insert, II, III exsert, and 



v 



90 



TEXT-FIGS. 88-92. Development of ocular plate arrangement in Ccnlrechinus setosm (Leske). 



88. Bahamas. Diam. 8 mm. R. T. J. Coll., 672. X 10. Oculars all strongly exsert, marginal plates of the peri- 

 proct are large, a youthful character, compare text-fig. 98. See p. 174. 



89. St. Thomas, West Indies. Diam. 33 mm. R. T. J. Coll., 755. X 5. Oculars still all exsert. 



90. The same. Diam. 33 mm. R. T. J. Coll., 756. X 5. Ocular I insert. 



91. The same. Diam. 31 mm. R. T. J. Coll., 757. X 5. Oculars I, V insert. 



92. The same. Diam. 31 mm. R. T. J. Coll., 758. X 5. Oculars I, V, IV insert. Ambulacra! plates simple dor- 

 sally above the 18th, which is the first compound plate (p. 55). 



this, though only 31 mm. in diameter, has taken on the full species character in this respect. 

 Interesting features of the corona are shown also in text-fig. 92. The ambulacral plates near 

 the ocular are all simple as in cidarids, the first compound plate being the 18th from the base 

 of the corona, as indicated. In the adult specimen (text-fig. 94) a similar condition of simple 

 plates is seen in the placogenous zone, and the first compound plate is the 24th from the base 

 of the corona. This condition of simple plates in the placogenous zone was pointed out by 

 Lovdn (1874), but is much clearer in Centrechinus than in many genera. The development 

 of the interambulacrum is shown in a similar manner in text-fig. 92; plates 11 and 12 are nearly 



