TETRAXOXIDA 



295 



Genus Sigmotylotella, gen.n. 

 Genotype. S. suberitoides, sp.n. 



Diagnosis. Mycaleae with main skeleton a confused reticulation of large tylostyli and 

 dermal skeleton a palisade of small tylostyli set more or less at right angles to surface ; 

 microscleres sigmata. 



Remarks. The structure of the skeleton in Sigmotylotella resembles that of Suberites 

 very closely, but the shape of the tylostyli and the presence of the sigmata definitely 

 show it to be a member of the Mycaleae. It seems to be most nearly related to Tylodesma. 



Sigmotylotella suberitoides, sp.n. (Fig. 20). 

 Holotype. B.M. 28. 2. 15. 400. 

 Occurrence. St. 6: Tristan da Cunha, 80-140 m. 



Diagnosis. Sponge encrusting; surface sparsely papillate, markedly hispid; oscules 

 situated at apices of papillae ; pores apparently distributed generally over surface ; in- 

 terior cavernous ; texture firm but friable ; colour, in spirit, black ; main skeleton a con- 

 fused reticulation of tylostyli of varying size arranged singly or in loose bundles ; dermal 



o fa c d e 



Fig. 20. Sigmotylotella suberitoides, sp.n. Tylostyli: a, normal; b, d, with 

 tuberculate base; c, with annulate shaft; e, developmental form. 



skeleton a dense palisade of small tylostyli directed more or less at right angles to sur- 

 face; tylostyli of main skeleton with curved shaft and well-developed spherical head, 

 I -o by o-oi8 mm. ; tylostyli of dermal skeleton similar in shape to those of main skeleton, 

 0-6 by 0-OI2 mm.; microscleres simple contort sigmata, 0-028 mm. chord. 



Remarks. A curious feature of the spicules is that, very often, the bases of the 

 tylostyli are tuberculate. This is more marked in the tylostyli of the dermal layer. 

 Further, it is significant that the bases of developing spicules are also tuberculate 

 (Fig. 20 e). Occasionally, a shaft may bear annular thickenings, and it is not improb- 

 able that this abnormality may be comparable with the tuberculations on the bases. 



Section lOPHONEAE 

 Genus lophon. Gray 



Alebion, Gray, 1867, p. 534 {nee Kroyer): preoccupied; lophon. Gray, 1867, p. 534 (genotype: 

 Halichondiia scandens, Bowerbank); Pocillon, Topsent, 1891, p. 539 (genotype: Halichondria 

 hyndmaiii, Bowerbank); lophonopsis, Dendy, 1924, p. 348 (genotype: Halichondria nigricans, 

 Bowerbank); Burtonella, de Laubenfels, 1928, p. 361 (genotype: Burtonella melanokhemia, de 

 Laubenfels). 



8-a 



