490 Herbert L. Hawkins — Studies on the Echinoidea. 



publication here than its seven predecessors have been. According 

 to Loven 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 Nacula among Irregular Echinoids. Although 

 one of the main purposes of the following pages is 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 Loven'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 

 Pygasteridaj must be phylogenetically ancestral to it. 



At the time of its description by Loven Pygastrides was almost 

 the only developmental stage of its kind known. Now there are 

 several comparable ontogenetic phases available for comparison, 

 notably the gnathostomatous young of Echinoneus cyclostotnus and the 

 originally endocyclic early post-larval forms of Abattis cavernosus and 

 Echinocardium flavescens. The three different ontogenetic lines thus 

 indicated serve to prove conclusively the phylogenetic relationship 

 of the Holectypoida to the Irregular Echinoids. Even if Echinoneus 

 be considered to be an Holectypoid (and I incline to believe that 

 it should be so classed), its aifinities with other groups are many 

 and manifest. Pygastrides, as the following arguments seek to show, 

 must be a young stage of some other type of Irregular Echinoid, 

 while Ahatus is a Spatangid of the Spatangoids. 



In the course of the discussion on the affinities of Pygastrides, 

 certain morphogenetic points arise. These are mainly concerned 

 with the perignathic girdle, so that this paper is in some respects 

 a direct sequel to its three immediate predecessors. It was thought 

 better to keep it distinct from them because, while they were based 

 upon direct observation, the substratum on which the following 

 arguments rest is theoretical, although it seems to me to be more 

 secure than mere conjecture. 



2. Resume of the Chaeacters of Pygastsides eelictus. 



In 1874 Loven [Etudes sur les Echinoidees, p. 79), in a footnote 

 to a description of certain " Echinoconidae " (Holectypoida), mentioned 

 the existence of a small recent form from the Caribbean Sea that 

 he believed to be a living species of Pygaster. He gave it the 

 nomen nudum of Pygaster relictus. After a delay of fourteen years, 

 during which "the little thing" succeeded in avoiding capture, in 

 spite of "most energetic exploration" of its habitat, Loven gave 

 a fall and beautifully illustrated description of the solitary and 

 imperfect specimen on which his previous comment had been bae d 

 (1888, " OnaEecentForm of the Echinoconidae "). Detailed study of 

 the small corona showed that it differed in many important respects 

 from that of Pygaster, and caused Loven to diagnose a new genus, 

 Pygastrides, for its reception. Pygastrides relictus still remains 

 unique, a sufficiently remarkable fact in view of the amount of 



