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



disposed in the sagittal plane. The ventral rod is strongly curved, often semicircular, the dorsal 

 rod less curved or nearly straight. 



Dimensions. Diameter of the gate 0'05 to O07 ; length of the spines 0'004 to 0'02. 



Habitat. Cosmopolitan Atlantic, Pacific ; also fossil in Barbados. 



11. Zygocircus biitschlii, n. sp. 



Zygocircus produdus, Biitschli, 1882, Zeitsehr. f. wiss. Zool., vol. xxxvi. p. 497. 



Grate obliquely ovate. King irregularly roundish, semi-ovate or ovate, with interrupted and 

 distorted edges, and with a large number (ten to twenty or more) of simple, irregularly formed and 

 asymmetrically disposed spines ; commonly some larger spines at the base. 



Dimensions. Diameter of the gate 0'05 to 0'09 ; length of the spines O'OOS to 0'02. 



Habitat. Fossil in Barbados. 



12. Zygocircus productus, Biitschli. 



Zygocircus productus, Biitschli, 1882, Zeitsehr. f. wiss. Zoo]., vol. xxxvi. p. 496. 

 Lithoeircus productus, R. Hertwig, 1879, Organismus d. Radiol., p. 69, Taf. vii. fig. 4. 



Gate obliquely ovate. Eing obliquely ovate, with three complete prominent edges and with 

 numerous (ten to twenty or more) simple, short conical spines, arising in three series from the three 

 edges ; commonly some smaller spines at the base. 



Dimensions. Diameter of the gate 01 to 0'2 ; length of the spines 0'005 to 0'02. 



Habitat. Cosmopolitan ; Mediterranean, Atlantic, Pacific, surface. 



Genus 404. Dendrocircus, 1 Haeckel, 1881, Prodromus, p. 447. 



Definition. S tephanida with a simple dipleuric or bilateral ring, armed with 

 branched spines, without typical basal feet. 



The genus Dendrocircus has the same dipleuric or bilaterally symmetrical form of 

 the sagittal ring as its ancestral genus Zygocircus. It differs from the latter in the 

 development of branched radial spines, and therefore bears to it the same relation as 

 the amphithect Lithoeircus does to the simpler Archicircus. 



1. Dendrocircus quadrangulus, n. sp. 



Gate irregularly quadrangular or nearly semicircular. Ring quadrangular, edgeless, with four 

 unequal sides ; ventral rod more curved and with longer sides than the dorsal rod. From the 

 four edges arise four strong, irregularly branched spines, about as long as the diameter of the gate, 



1 De7idroaYctts=Ring bearing trees ; 8si/8jo, xloxo;. 



