and Embryological Development. 353 



Although in Hemicidaris the number of the coronal plates 

 has increased as compared with the Cidaridse, and while we 

 find that in many genera, even of those of the present day, 

 the number of the coronal plates is still comparatively small, 

 yet, as a general rule, the more recent formations contain 

 genera in which the increase in number of the interambulacral 

 plates is accompanied by a corresponding decrease in the 

 number of plates of the interambulacral area, so characteristic 

 thus far of the Cidarida3 and Hemicidaridse, a change also 

 affecting the size of the primary ambulacral tubercles. This 

 increase in the number of the coronal plates is likewise 

 accompanied by the development of irregular secondary and 

 miliary tubercles, and the disappearance in this group of the 

 granular tuberculation, so important a character in the Cida- 

 rida3. With the increase in the number of the interambulacral 

 coronal plates, the Pseudodiadematida3 still retain prominent 

 primary tubercles, recalling the earlier Hemicidaridaj and 

 Cidaridaj, and, as in the Cidaridae proper, the test is fre- 

 quently ornamented by deep pits or by ridges formed by the 

 junction of adjoining tubercles. The genital ring becomes 

 narrower, aud the tendency to the specialization of one of 

 its plates, the madreporite, more and more marked. 



With the appearance of Stomechinus, the Echinida? proper 

 already assume in the Jura the open arcs of pores, the large 

 number of coronal interambulacral plates, the specialization 

 of the secondary tubercles, and the large number of primary 

 tubercles in each plate. With the appearance of Sphozre- 

 chinus in the early Tertiary come in all the elements for 

 the greater multiplication of the pairs of pores in the arcs of 

 the poriferous zones, while the gigantic primary spines of 

 some of the genera (Heterocentrotus) and the small number 

 of primary tubercles are structural features which had com- 

 pletely disappeared in the group preceding the Echiuome- 

 tradas, to which they appear most closely allied. 



Going back again to the Hemicidarida3, it requires but 

 slight changes to pass from them to Acrosalenia and to the 

 Salenice proper ; the latter have continued to the present day, 

 and have, like the Cidaridce, retained almost unchanged the 

 characters of the genera which preceded them, combined, 

 however, with a few Cidaridian and Echinid featm - es which 

 date back to the Triassic period. We can thus trace the 

 modifications which have taken place in the poriferous zone, 

 the apical and actinal systems, the coronal plates, the ambu- 

 lacral and interambulacral tubercles, as well as in the radioles, 

 and, in the most direct manner possible, indicate the origin of 

 the peculiar combination of structural features which we find 



