768 



THE VOYAGE OF H.M.S. CHALLENGER. 



less compressed from both poles of the spineless axis, lenticular or discoidal, sometimes 

 square. It is enveloped by a voluminous calymma constantly bearing coronals of 

 " Myophrisca " (compare p. 724). 



Synopsis of the Genera of Quadrilonchida. 



I. Subfamily 



Acanthostaurida. 

 All twenty spines simple, 

 without lateral apophyses 

 (sometimes forked, but 

 neither branched nor 

 latticed). 



II. Subfamily 



Lithopterida. 



Either all twenty spines or 

 a part of them provided 

 with two opposite lateral 

 branches or apophyses. 



Four equatorial spines of 

 equal size and form. 



Four equatorial spines of 

 very different size or 

 form (the two lateral 

 constantly equal). 



Eight tropical and eight 

 polar spines nearly 

 equal, . 



Eight tropical and eight 

 polar spines very 

 different, 



f Two principal spines of 

 equal size and form, 



Two principal spines 

 (frontal and caudal) 

 very different, . 



Apophyses simple, neither branched nor latticed, 



1- Apophyses branched or pinnate, but not latticed, 



Apophyses latticed, with fenestrated network, . 



334. Acanthostaunis. 



335. Belonostaurus. 



336. Lonchostaurus. 



337. Zyostuurus. 



338. Quadrilonche. 



339. Xiphoptera. 



340. Lithoptera. 



Subfamily 1. ACANTHOSTAURIDA, Haeckel, 1881, Prodromus, p. 466. 

 Definition. Q uadrilonchida with simple radial spines, without apophyses. 



Genus 334. Acanthostaurus, 1 Haeckel, 1862, Monogr. d. Radiol., p. 395. 



Definition. Q uadrilonchida with four equatorial spines of equal size and 

 form, which are much larger than the sixteen other spines. Eight tropical and eight 

 polar spines nearly equal. No apophyses. 



The genus Acanthostaurus is the most simple and primitive form of the Quadri- 

 lonchida, and the common ancestral genus of this family ; it is at the same time its most 

 common and widely distributed form. Some species appear in astonishing numbers in 

 different seas. It has been derived from Acanthometron by stronger development of 

 the four equatorial spines, which are all of equal size and much larger than the sixteen 

 others. 



1 A canthostaurus Spine-cross ; xctt/6tt, orat/jof. 



