444 M. C. Mereschkowsky on new Hydroida 



at the same time, polyserial arrangement of the hydrothecae. 

 The figures 1-3 (p. 443) will explain what I have just said. 

 Fig. 1 represents a biserial type ; fig. 2 is a polyserial type, 

 produced by the alternate displacement of the hydrothecge of 

 the same series, one to the left, the other to the right. The 

 round spaces, of which the surface is shaded, represent the 

 hydrothecae belonging to the same series. It is thus that the 

 first series in fig. 1 has divided into the first and second series 

 in fig. 2. Lastly, fig. 3 represents the same polyserial type, 

 but arising in this case from the torsion of the series and their 

 position in a spiral. In fact all the species (as for example, 

 S. mrrabtlis, 8. Hincksii, S. ochotensis^ and S. decern serialis) 

 which have many series, and even those which have only a 

 few (as for example, 8. pinnata^ 8. pacijica, and even 8. tri- 

 serialis), show a very marked spiral arrangement, just as 

 represented in the diagrammatic figure 3. If my explanation 

 of the origin of the polyserial type by torsion were not correct, 

 if it were necessary to accept the other explanation, we should 

 also have to expect that, on the contrary, the arrangement 

 represented in fig. 2 would predominate in the difl:erent species 

 — which, however, is not the case ; the spiral arrangement, 

 as also the fact that all the apertures of the hydrothecae in 

 this genus are placed at different heights, cannot be ex- 

 plained without accepting the explanation that I have given 

 above. 



Length of the largest colony of 8elaginopsis decemserialis 

 180 millims., breadth (maximum) 25 ; breadth of a branch 

 1*25 ; length of the hydrotheca 0*45, its breadth 0*25. 



The museum of the Academy of St. Petersburg possesses 

 several examples of this species, which differs from all the 

 others by the decemserial arrangement of its hydrothecae, as 

 well as by its habit and mode of ramification. 



I now proceed to mention all the other species of this genus 

 at present known, which, together with those just described, 

 constitute a considerable number. 



7. 8elaginops{s HincksUj mihi. 



Polyserias glacialis, Mereschk. Ann. & Mag. Nat. Hist. ser. 4, vol. xx. 



(1877), p. 228. __ 

 Pulyserias Hincksii, Mereschk. Ann. & Mag. Nat. Hist, ser, 5, vol. i. 



(1878), p. 337, pi. XV. figs. 1-4. 



Colony plumiform, very large and broad ; branches simple, 

 springing alternately from all sides of the stem ; hydrothecae 

 entirely immersed in the substance of the axial tube, ar- 



