398 H. L. Hawkins — Echinoidea Holectyiioida. 



appear to oscillate (save perhaps for steady reduction in the 

 expansion of the petals), phyllode-development seems to show 

 progressive (or, perhaps, regressive) modification. The plate- 

 complexity that was concentrated into the true phyllodes of the 

 older Nucleolitids (such as Galeropygus and " Nucleolites " quadratus) 

 spread gradually away from the peristomial region until in area III 

 of Apatopygus it involves the greater part of the area in triad- 

 grouping. As far as phyllode-production is concerned, the change 

 was catagenetic, but in respect of the complexity of the whole 

 ambulacrum it was clearly anagenetic. 



The sequence of ambulacral morphogenesis indicated above was 

 carried out in a series of forms whose general evolution has been 

 relatively static. Save for its ambulacral plating, Apatopygus 

 would not have been out of place on a Middle Jurassic beach. 

 Another line of descent from the Galeropygus-N ucleolites stock led 

 in quite an opposite direction, and produced the Clypeus-Pygurus 

 series with increasingly complex and limited phyllodes. That 

 essentially Mesozoic group showed far more diversity in form, size, 

 and detail than the conservative Nucleolitidse, and paid the penalty 

 of over-exuberance by extinction in the Cretaceous period. The two 

 Nucleolitoid series thus provide clear illustration of the principles 

 of evolution already familiar in many phyla. In comparison with 

 Brachiopoda, Apatopygus may be said to bear a similar relation to 

 Galeropygus to that borne by Lingula to Linguhlla ; while Pygurus 

 would agree more with Spirifer. The persistent Nautilus and the 

 extinct " Ammonites " have comparable histories. 



The ambulacra of Apatopygus invite comparison with those of the 

 Echinoneidas. Save for the presence of petals and " biporous " 

 orad ambulacrals, A. recens has typical Pyrinid ambulacral plating, 

 almost perfect as far as it extends. In area I (PI. VII, Fig. 1) 

 there occur three Discoidiid triad-groups (similar plates appear 

 in Trematopygus), so that the correspondence with Holectypoid 

 structures is emphasized. It seems quite inconceivable that any 

 phyletic link can connect Apatopygus with Pyrina or any other late 

 Holectypoid ; so that the appearance of Pyrinid plating in the 

 recent form must be ascribed to parallel development. 



If orthogenesis supplies an explanation of this coincidence, the 

 separable qualities of the components of " individuals " is strikingly 

 illustrated. The ambulacra of Apatopygus have come to conform 

 to the Holectypoid standard in essentials (albeit by a non- 

 Holectypoid route) ; while other coronal structures have followed 

 quite different lines of development. Further, the appearance of 

 Pyrinid plating in the last of the Nucleolitoida seems to give support 

 to the belief that that stock branched from the Holectypoid series 

 (though early) rather than arose independently. That extensive 

 triad-grouping or combination after this pattern appeared in Lower 

 Cretaceous times in the Echinina, Echinoneidae, Lanieriidae, and 

 Nucleolitidee, is a coincidence explicable only on the assumption 



