It may be thought that some apology is due to the readers of this Magazine on the ground of the largely zoological bearing of this paper. But in the opinion of the writer no such apology is necessary. Zoology is palæontology brought up to date, and ontogeny is but a compressed and individualistic type of phylogeny; so that in spirit, though not in matter, this paper is no less appropriate for publication here than its seven predecessors have been. According to Lovén himself, the minute Echinoid which supplies the text for the sequel is to be considered a survivor, not merely of the Holectypoida, but of the most primitive family of that Order—a kind of Lingula or Nucula among Irregular Echinoids. Although one of the main purposes of the following pages ia to offer reasons for disbelieving that contention, nevertheless Pygastrides, as far as the only specimen known is concerned, is for all practical purposes primitively Holectypoid in essential characters. This seeming paradox may be resolved by the anticipatory remark that Lovén's so-called genus is believed to be an early post-larval stage in the development of some more completely Irregular adult form. It is ontogenetically related to that problematical adult, just as the Pygasteridæ must be phylogenetically ancestral to it.