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



13. Amphipyle callizona, n. sp. (PI. 9, fig. 6). 



Cortical shell smooth, with sixteen strong and short conical spines, lying opposite in pairs in 

 two crossed diagonal planes ; each lateral wing four-sided prismatic, its lateral face concave, both 

 ends truncated, and each end provided with four divergent spines. Length of the wings some- 

 what greater, but breadth smaller, than that of the medullary shell. 



Dimensions. Length of the medullary shell O07, breadth 0'05 ; length of the lateral wings 

 0-09, breadth 0'03. 



Habitat. Western Tropical Pacific, Station 225, depth 4475 fathoms. 



Genus 283. Tetrapyle, 1 J. Miiller, 1858, Monatsber. d. k. preuss. Akad. 



d. Wiss. Berlin, p. 154. 



Definition. Pylonida with trizonal lentelliptical medullary shell, surrounded 

 by two crossed latticed cortical girdles, one smaller (primary) transverse, and one 

 larger (secondary) lateral girdle. Four gates between the two cortical girdles simple, 

 without a sagittal septum. 



The genus Tetrapyle, till 1881 the only known genus of the whole family, was 

 founded by Johannes Miiller in 1858, and clearly illustrated by the Mediterranean (and 

 common cosmopolitan) Tetrapyle octacantha, hitherto the best known type of this 

 family. Afterwards (1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 832) 

 Ehrenberg founded the genus Schizomma for a nearly allied form, which exhibits only 

 slight specific differences (compare my Monograph, 1862, p. 434). Some good remarks 

 on the structure of this typical genus and its relations to other Pylonida are to be 

 found in Richard Hertwig's Organismus, &c., 1879, p. 52, but the true trizonal 

 structure of the medullary shell in this genus was not recognised by him, so that his 

 descriptiori agrees more with Dizonium. We confine here the genus Tetrapyle to those 

 Pylonida rpr which Tetrapyle octacantJia of J. Miiller remains the determining type ; 

 the cortical shell is composed only of two perfect lattice-girdles (the transverse and 

 lateral), between which four wide gates remain open. This structure is similar to 

 that of Dizonium ; but whilst here the medullary shell is a simple central chamber, in 

 Tetrapyle it is a complete trizonal or Larnacilla-shcll. 



Subgenus 1. Tetrapylissa, Haeckel. 



Definition. Cortical shell smooth or thorny, but without large, symmetrically 

 disposed spines. 



1 Tetrapyk = With four gate-openings ; 



