and Embryological Development. 361 



ambulacral system in order to become petaloid, and the 

 gradual change of the elliptical ovoid test of the young to 

 the characteristic generic test, accompanied by the rapid in- 

 crease in the number of the primary tubercles and spines. 

 Finally, limited to the Spatangoids are the changes they 

 undergo in the transformation of the simple actinostome 

 to a labiate one, the specialization of the anterior and 

 posterior parts of the test, and the definite formation of the 

 fascioles. 



Comparing this embryonic development with the paleeon- 

 tological one, we find a remarkable similarity in both, and in 

 a general way there seems to be a parallelism in the appear- 

 ance of the fossil genera and the successive stages of the 

 development of the Echini as we have traced it. 



We find that the earlier regular Echini all have more or 

 less a Cidaris-like look (that is, they are Echini with few 

 coronal plates, large primary tubercles with radioles of a 

 corresponding size) , that it is only somewhat later that the 

 DiademopsidaB make their appearance, which, in their turn, 

 correspond within certain limits to the modifications we have 

 traced in the growth of the young Diadematidge and Arba- 

 ciada3. The separation of the actinal system from the coronal 

 plates has. been effected. The poriferous zone has either 

 become undulating or forms somewhat indefinite open arcs ; 

 we find in all the genera of this group a larger number of 

 coronal plates, more numerous primaries, the granules of the 

 Cidaridffi replaced by secondaries and miliaries, and traces of 

 a Hemicidaris- like stage in the size of the actinal ambulacral 

 tubercles. 



Comparing in the same way the palgeontological develop- 

 ment of the Echinidas proper, we find that, on the whole, 

 they agree well with the changes of growth we can still follow 

 to-day in their representatives, and that, as we approach 

 nearer the present epoch, the fossil genera more and more 

 assume the structural features which we find developed last 

 among the Echinidas of the present day. Very much 

 in the same manner as a young Echinus develops, they 

 lose, little by little, first their Cidaridian affinities, which 

 become more and more indefinite ; next their Diadematidian 

 affinities, if I may so call the young stages to which they 

 are most closely allied ; and finally, with the increase in the 

 number of the coronal plates, the great numerical development 

 of the primary tubercles and spines, and that of the seconda- 

 ries and miliaries which we can trace in the fossil Echini of 

 the Tertiaries, we pass insensibly into the generic types 

 characteristic of the present day. 



Ann. &Mag. N. Hist. Ser. 5. Vol. vi. 26 



