436 M. C. Meresclikowsky on new Hydroida 



This is a very characteristic species, and may be easily 

 recognized by the triserial arrangement of its hydrothecse, 

 which produces the extreme fineness of its branches. In the 

 mode of its ramification and the form of its hydrothecse it does 

 not differ greatly from the other species. As all the other 

 species have 4, 6, 8, or more longitudinal series of hydrothecge, 

 Selaginopsis tnseriah's, having only three, must consequently 

 be regarded as the simplest form. 



It might be supposed that the number of series in the poly- 

 serial type had originated from the biserial type by the dis- 

 placement of the hydrothecre in the two series alternately to 

 one side and the other, which would produce the division of 

 one series into two, as indicated by Selaginopsis fusca, Norm., 

 and S. Allmani, Norm., in which the series are arranged in 

 pairs — which would be the most plausible and natural expla- 

 nation. Unfortunately Selaginopsis triserialis only serves to 

 throw the question into confusion ; for this species can by 

 no means be explained as having originated from the biserial 

 type. 



2. Selaginopsis pinnata^ sp. n. (PI. XVI. figs. 3, 4.) 



Trophosome. Hydrorhiza in the form of a thin and continu- 

 ous layer, not composed of hydrophytons. Hydrocaulus simple, 

 straight, not angularly bent, annulated at the base, divided 

 into regular internodes. Branches pinnate, straight, springing 

 alternately from two sides, not divided into internodes. 

 HydrothecjB arranged in four regular series, almost entirely 

 immersed in the tubular axis ; they do not follow one another 

 immediately in the same series, but leave a certain interval 

 between them. Their form is cylindrical, a little narrowed at 

 the end, with a very short neck, springing outward from the 

 axis ; aperture oval, very slightly angular at the two sides. 



Gonosome. Unknown. 



Locality. Port Ajan [M. Wosnessensky, 1848). 



The hydrorhiza is formed by a thin layer covered with 

 perisarc (PI. XVI. fig. 3, per), and not composed of tubes 

 united together, as described by me in Sertularta alhimaris ; 

 here, on the contrary, the perisarc is continuous, covering, on 

 shells, spaces sometimes of more than a square centimetre. 

 The hydrocaulus is erect, stout, cylindrical, very long, divided 

 into regular internodes about 8 millims. in length, broader 

 than the branches ; its colour is a darkish brown, becoming 

 lighter towards the end, where the branches commence, which 

 are also light brown. Usually only one half or one third, or 

 sometimes even one fourth, of the stem is covered with 

 branches, all the rest being entirely destitute of them (PI. XVI. 



