TETRAXONIDA 337 



Genus Tentorium, Vosmaer. 



Tentorium semisuberites (Schmidt). 



Thecophora semisuberites, Schmidt, 1870, p. 50, pi. vi, fig. 2; T. ibla, Thompson, 1873, P- ^4^' 

 fig. 24; T. semisuberites, Whiteaves, 1874, p. 9; T. ibla, Verrill, 1874, p. 500, pi. viii; T. semi- 

 suberites, Vosmaer, 18S5, p. 18, pi. i, figs. 23, 24, pi. iii, figs. 22-26; Tentorium semisuberites, id., 

 1885, p. 329, pi. ii, fig. 4, pi. xxi, fig. 19 ; Ridley and Dendy, 1886, p. 489 ; id., loc. cit., p. 221 ; 

 Thecophora semisuberites, Fristedt, 1887, p. 433; Tentorium semisuberites, Lambe, 1896, p. 198, 

 pi. iii, fig. 2; id., 1900, p. 25; Arndt, 1912, p. 113 ; Topsent, 1913, p. 25 ; Ferrer, 1914, p. 19. 



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

 Distribution. Arctic and Atlantic Oceans. 



Genus Polymastia, Bowerbank 



Polymastia isidis, Thiele (Fig. 39). 



P. isidis, Thiele, 1905, p. 414, figs. 25, 38 a~e; P. isidis, var. simplex, Hentschel, 1914, p. 47, 

 pi. V, fig. 3. 



Occurrence. St. 187: Palmer Archipelago, 259 m.; St. WS81: Falkland Islands, 81-82 m.; 

 St.WS82: Falkland Islands, 140-144 m. ; St.WS86: Falkland Islands, 151-147 m.; St.WS248: 

 Falkland Islands, 210-242 m.; St. WS 250: Falkland Islands, 251-313 m. 



Remarks. The species is represented by nearly a dozen specimens, all agreeing closely 

 with each other, but differing slightly from the holotype in external form and minor 

 details of the spiculation. They are all cushion-shaped, standing on a circular base, and 

 the upper surface bears a varying number of wart-shaped 1 1 

 papillae. The papillae are never elongate, as in the type, but 

 always low, not exceeding 2-3 mm. in height, and in some not 

 perceptibly raised above the level of the surrounding surface. 

 The chief difference in the spiculation is the presence of a 

 pecuUar annulation on many of the large tylostyles. The nor- 

 mal base of these tylostyles is as shown in Fig. ^q c, but more „. „ , . . .,. 

 ■' ■' o oy J |.jg ^g Polymastia isidis, 



than 50 per cent have a base such as that shown in Fig. 39 ^>, Thiele; showing variations 

 while Fig. 39 a represents the condition in about 10 per cent, in shape of bases of mega- 

 Many of these spicules are polytylote, as in the type, but scleres. x 300. 

 wherever this occurs the swellings on the shafts are of the kind shown in Fig. 39 a. 



At first sight, it would appear that the differences between the specimens here de- 

 scribed and the type of the species are sufficient to justify the recognition of a new 

 variety, but the general similarity in te.xture, appearance of the surface, disposition of 

 the spicules in the skeleton, and the shape of the normal megascleres make it fairly 

 certain that the Discovery specimens represent at the most a tropus (sensii Vosmaer) 

 of P. isidis. 



Making this more certain, there are a further nine specimens in the collection which 

 resemble closely in external appearance the dozen specimens referred to above. These 

 are mostly cushion-shaped, but one is irregularly massive and one sub-spherical. The 



