TRIGONOCIDARIS ALBIDA. 289 



Similar changes of growth are observed in Temnopl. Hardwickii (PI 

 VIII. f. 25, 26). The secondary tubercles appear at first like radiating 

 spokes. The pits are sharp, rectangular (PI. VIII. f. 25), while in somewhat- 

 older stages they are more circular (PL VIII f. 26), the secondary tubercles 

 isolated, and eventually these pits become mere smooth, bare, angular spaces 

 at the junction of the plates in the adult. The increase in number of plates 

 of the anal system, due to age, is seen in comparing PI. VIII. f. 2S and PI. 

 VIII. f. 27 ; the anal system undergoing changes similar to those of the 

 anal system of Toxopneustes, Echinus, and the like, where no one plate re- 

 tains a greater prominence, as is so markedly the case in Temnopleurus, where 

 the plate, originally covering the whole anal system, is peculiarly orna- 

 mented, and retains throughout its characteristic features, and is readily 

 distinguished (PI. VIII. f. ;.'/) in the oldest specimens from the plates sub- 

 sequently added in younger specimens. 



D'Archiac and Haime have figured from the Nummulitic formation of India 

 a number of species, which are usually referred either to Temnopleurus or 

 to Opechinus, which belong to this same genus Temnechinus. 



TRIGONOCIDARIS. 



Trigonocidaris A. Agass. 18G9. Bull. M. C. Z., I. 



Small Echini, test thin, regularly arched above, depressed below ; actinal 

 cuts slight, actinal membrane strongly imbricated. Two principal vertical 

 rows of tubercles, both in the ambulacral and interambulacral areas ; second- 

 aries and primaries connected by ridges raised above test, forming an irreg- 

 ular network of ridges, with more or less deep pits between them ; porifer- 

 ous zone narrow, pores arranged in single vertical pairs, spines longitudinally 

 and laterally striated. Abactinal system large, genital plates excluding 

 smaller ocular plates from anal system, which is covered by four plates. 



Trigonocidaris albida 



! Trigonocidaris albida A. Ag 1869. Bull. M. C. Z., I. 



PI. IV. f. 1-7. 



Small species, in which the primary tubercles have the same structure as in 

 Temnechinus ; but, in addition, the whole test is covered by a reticulation 

 of ridges, similar to those of Podocidaris, but more numerous and quite 



