808 THE VOYAGE OF H.M.S. CHALLENGER. 



Su1)family 1. Dipoeaspida, Haeckel. 



Definition. — Dorataspida with twenty radial spines, each of wliieli bears two 

 opposite apophyses. The spherical shell is composed either of the meeting branches of 

 these apophyses (Phractaspida), or of twenty perforated plates, produced by concrescence 

 of their branches (Ceriaspida). 



A. Tribe I. P h r a c t a s p i d a, Haeckel. 



Definition. — Dorataspida without perforated plates; the spherical shell is 

 composed only of the meeting branches of the two opposite apophyses, which arise from 

 each radial spine. Therefore the meshes of the shell are all sutural. 



Genus 349. Phractaspis,^ Haeckel, 1881, Prodromus, p. 467. 



Definition. — Dorataspida without perforated plates; shell composed only of 

 the meeting branches of the two opposite apophyses, which arise from each radial spine. 

 Condyles of the branch-ends without l)y -spines. 



The genus Phractaspis is the most simple and primitive form among all Dorata- 

 spida, and may be regarded as the common ancestral form either of this whole family, or 

 at least of its first subfamily, the Diporaspida. In all members of this subfamily the 

 spherical shell is composed of twenty radial spines, each of which bears two opposite 

 apophyses ; but the mode of composition is different in the two tribes of the subfamily, 

 in the Phractaspida and Ceriaspida. In the simpler tribe, the Phractaspida, the shell 

 is composed only of the meeting branches of the apophyses of neighbouring spines ; 

 thei;e are no peculiar perforated plates or shields. In the Ceriaspida, however, both 

 apophyses of each single S23ine form a perforated plate or shield l^y union of their 

 branches, and the shell is formed of the meeting edges of these shields. Of course 

 the Ceriaspida must l)e derived from the simpler Phractaspida. Phractaspis, as 

 the common ancestral form of both, exhibits a very simj^le structure of the shell 

 (PI. 137, figs. 1, 2). Commonly, if the fork -branches of each apophysis be not again 

 branched, the shell possesses only twenty -two large meshes and forty sutures. More 

 rarel}^ their number increases, the foi'k -branches of the apophyses being again branched 

 {Ph ractaspidium, PI. 137, fig. 3). 



Subgenus 1. Phractasparium, Haeckel. 



Definition. — Shell with twenty-two meshes, and forty sutures, each spine with only 

 four liranches, its two apophyses being simply forked. 



' Phractaspis — Heiigmg shield ; tpt-xuvd:, ai-i=. 



