908 



THE VOYAGE OF H.M.S. CHALLENGER. 



I. Subfamily 

 Triplagicla. 

 Three radial sijines. 



IL Subfamily 



Tetraplagida. 

 Four radial spines. 



III. Subfamily 



Hexaplagida. 

 Six radial spines. 



IV. Subfamily 



Polyplagida. 

 Numerous radial spines. 



Synopsis of the Genera of Plagonida. 



Three spines lying in one horizontal plane, . 

 Three spines corresponding to the edges of a flat pyramid, 



All four spines equal, 



Four spines arising from one 

 common central point. 



Four spines arising in two 

 pairs from the poles of a 

 common central rod. 



One apical spine opposed to 



three basal spines, 



All four spines equal, 



One apical spine opposed to 

 tlu'ee basal spines. 



(" Six spines arising from one common central point, 



J 



Six spines arising in two opposite groups from the poles of a 

 common central rod, ..... 



Numerous (seven to nine or more) radial spines arising from 

 a common centre (either a central point or a branched 

 rod), ....... 



384. Triplagia. 



385. Plagiacantha. 



386. Tetraplagia. 



387. Plagoniscus. 



388. Plagonidium. 



389. Plagiocarpa. 



390. Flexaplagia. 



391. Plagonium. 



392. Polyplagia, 



Subfamily 1. Tkiplagida, Haeckel, 1881, Prodromus, p. 423. 

 Definition. — P 1 a g o n i cl a with three radial spines. 



Genus 384. Triplagia,^ Haeckel, 1881, Prodi-omus, p. 423. 



Definition. — P lagonida with three radial spines, arising from one common 

 central point and lying in one horizontal plane. 



The genus Triplagia and the following closely allied Plagiacantha may be 

 regarded as the simplest and most primitive forms of the Plectoidea, perhaps as 

 the common ancestral stock of this suborder. The skeleton is composed of three simple 

 or branched radial spines, arising from one common central point. These three spines 

 in Triplagia lie in one and the same plane, whilst in Plagiacantha they lie in different 

 planes. Therefore the former exhibits the simplest type of the triradial structure, 

 common to the majority of Nassellaria. 



' JVipZa(/ia= Triangular or three-radial ; Tg/xAay/of. 



