and Embryological Development, 363 



of the group become more prominent features of the suceecl- 

 ing genera. Accompanying the persistent type Echinobrissus, 

 genera appear in which either the bourrelets or petals have 

 undergone modifications more extensive than those of the 

 same parts in the genera of the Echinoneus or Caratomus 

 type. 



The earliest Spatangoids belong to the Dysasteridae, appa- 

 rently an aberrant group, but which, from the history of the 

 young Hemiaster, we now know to be a strictly embryonic 

 type, which, while it thus has affinities with the true Spatan- 

 goids, still retains features of the Cassidulidaj in the mode of 

 development of the actinostome and of the petals, as well as 

 of the anal system. The genera following this group, Holas- 

 ter and Toxaster, can be well compared, the one to the young 

 stages of Spatangus proper before the appearance of the petals, 

 when the ambulacra are flush with the test, and when the test 

 is more or less ovoid, the other to a somewhat more advanced 

 stage, when the petals have made their appearance as semi- 

 petals. In both cases the actinostome has the simple struc- 

 ture characteristic of all the young Spatangoids. The changes 

 we notice in the genera which follow them lead in the one 

 case through very slight modifications of the abactinal sys- 

 tem, of the anterior and posterior extremities of the test, to 

 the Ananchytid-like Spatangoids of the present day, the Pour- 

 talesice, the genus Holaster itself persisting till well into the 

 middle of the Tertiary period ; while, on the other side, we 

 readily recognize in the Spatanginas which follow Toxaster 

 (a persistent type which has continued as Palceostoma to the 

 present day) the genera which correspond to the young stages 

 of such Spatangoids as Spatangus and Brissopsis of the pre- 

 sent day — genera which, on the one hand, lead from Hemiaster 

 (itself still represented in the present epoch), through stages 

 such as Cyclaster, Peripneustes , Brissus, and Schizaster, and, 

 on the other, through Micraster and the like, to the Spatan- 

 goids, in which the development of the anal plastron and 

 fasciole performs an important part, while in the former 

 group the development of the peripetalous fasciole and of the 

 lateral fasciole can be followed. None of the genera of Peta- 

 losticha belonging to the other groups develops any fasciole in 

 the sense of circumscribing a limited area of the test. 



The comparison of the genera of Echini which have ap- 

 peared since the Lias with the young stages of growth of the 

 principal families of Echini, shows a most striking coinci- 

 dence, amounting almost to identity, between the successive 

 fossil genera and the various stages of growth. This identity 

 cannot, however, be traced exactly in the wav in which it 



26* 



