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



large as the two spines of the transverse axis. Eight troisical spines of similar form, but only half 

 as large, and with a very large, extremely prominent, lamellated leaf-cross. Eight polar spines quite 

 rudimentary, scarcely prominent. 



Dimensions. — Length 0'36, breadth 0'2. 



Habitat. — Central Pacific, Station 271, surface. 



Genus 337. Zijgostcmrus,^ n. gen. 



Definition. — Q u a d r i 1 o n c li i d a with four equatorial spines of unequal size and 

 form ; both lateral spines (in the transverse axis) equal ; but Ijoth principal spines 

 (in the longitudinal axis) very different, the frontal larger than the caudal. Sixteen 

 other spines miich smaller (often the eight tropical larger than the eight rudimentary 

 polar spines). No apophyses. 



The genus Zygostaurus differs from all other Staurolonchida in the peculiar 

 differentiation of the four equatorial spines ; the two opposite spines of the hydi-o- 

 tomical or longitudinal axis being very different in size and form (the frontal spine 

 forked, the caudal spine simple); whilst the two opposite spines of the geotomical or 

 lateral axis (j^erpendicular to the former) are equal, forked, but different in shaj)e from 

 the former. Therefore the geometrical fundamental form of the liody in this remark- 

 able genus becomes " amphithect " or " bilateral " in the widest signification of this 

 term (comp. my General Morf)hology, vol. i. pj). 480, 482). Of the three different 

 dimensive axes one (the longitudinal) exhibits unequal poles, whilst the two other (the 

 .sagittal and lateral) axes exhibit equal poles. 



1. Zygostaurus amphithectus, n. sp. (PI. 131, fig. 7). 



Frontal spine (cl) little different from the two lateral spines ; each with two divergent curved 

 horns, which are equal and of about the same length as the simple broad basal part. Caudal spine 

 (c.3) simple, spindle-shaped, about as long as the frontal spine. Tropical sjiines little smaller than 

 the three former, and of the same symmetrical forked form. Eight polar spines much smaller, but 

 also forked, with thm bristle-shaped branches. 



Dimensions. — Length of the cross (longitudinal axis) 0"5, breadth (lateral axis) 04. 



Habited. — Central Pacific, Station 272, surface. 



2. Zygostaurus longicornis, n. sp. 



Frontal spine (cl) little different from the two lateral spines, each with two divergent curved 

 horns, which are equal and of about the same length as the simple broad basal part. Caudal 



^ ^!/^osteunts= Pair-cross ; ^vyou, oTau^o;. 



