820 MR. J. RITCHIE OX HTDROIDS [May 24, 



between the liydi-othecse. These are connected at tlie base of 

 each hydrotheea by a strong bridge of coenosarc, upon the middle 

 of wliich the base of the hydranth rests. These structures 

 closely resemble those which Nutting has observed in Selaginopsis 

 ornata (1904, p. 7), allowing for the differences in symmetry due 

 to the presence of four series of hydrothecse in that species, in 

 place of two in this. Although each tube possesses a thick 

 definite ectoderm, I have been unable to distinguish the perisarc 

 which, Nutting supposes, surrounds each coenosarcal tube. 



The structui'e of the polyp is like that of Thuiaria rohusta. as 

 figured by Nutting (1904, p. 11, fig. 15). It has a large blind 

 sac, and clumps of large enilodermal cells project into the 

 hydranth cavity except from the abcauline wall of the blind-sac, 

 where the development of the endoderm is insignificant. The 

 protractor band is attached, not to the abcauline wall of the 

 hydrotheea, but to varying points of the lateral wall in the 

 neighbourhood of the margin of the hydrotheea. 



The gonosome was not observed. 



Dimensions : — 



Stem, diameter 0"56-0'77 mm. 



Stem internode, length 2 mm. 



Hydrotheea, length adnate 0'46-0'49 mm. 



„ free 0-07-0-14 mm. 



,, diameter at mouth 0'21-0'22 mm. 



Localities. St. 1, east of Tavoy Island and Port Owen, 4 to 12 

 fathoms, sand and broken shells, and mud ; several fragments. 

 St. 23, Five Islands, 8 to 12 fathoms, rock and .sand, and mud; 

 one colony. St. 25, Gregoiy Group and Crichton Island, 4 to 14 

 fathoms, stones and broken shells, and rock ; one colony. 



Previously known only from Eastern and Northern Australia ; 

 Port Curtis and Holborn Island (Bale, 1884), Peel Island, 

 Moreton Bay (Bale, 1888), Flinders Passage, Torres Strait 

 (Allmau, 1888, as Thuiaria vincta*), Thui-sday Island (Weltner, 

 1900). 



Idia pristis Lamouroux, 1816. 



One of the commonest of the species in the collection, found at 

 eight of the fourteen stations at which Hydroids were obtained. 

 There is much variability in the length of the free portion of the 

 hydrotheea, the sloping operculum appearing, in some cases, to 

 project directly from the surface of the stem, while in others a 

 large part of the hydrotheea stands out at right angles to the 

 stem. Occasionallj', too, the adcauline operculum is lacking in 

 definiteness, the upper wall of the hydrotheea simply meeting 

 the lower as an indefinite flap. Many of the branches from 

 Station 35 end in stolons, indistinguishable from those which 

 project from the hydrorhizal tubes. 



* See Billard, 1908, p. 2 of separate copy. 



