446 M. C. Mereschkowsky on neio Hydroida 



I here give a dichotomic Table which will facilitate the 

 determination of all the species of the genus Selaginopsis'. — 



1. Branches springing from two sides only of the stem 



(colony pinnate) 2. 



Branches springing ifrom all sides of the stem (colony 

 of the form of TJwjaria thuja^ except S. triserialis) . 8. 



2. Branches subdividing into branchlets 3. 



Branches simple, not subdividing 4. 



3. HydrotheciB arranged in four series ; branches not 



wide 'S'. pacifica. 



Hydro thecse in from four to six series ; branches 



broader, cylindrical 8. cylindrica. 



4. Hydrothecfe arranged in six series 5. 



Hydrothecae in four series 6. 



6. HydrothecjB entirely immersed in tlie substance of 



the branch ; margins of the aperture smooth . . S. Hincksii. 

 Upper part of hydrothecse projecting; aperture with 



two teeth S. mirahilia. 



6. The four series arranged in pairs 7. 



The four series not arranged in pairs 8. pinnata, 



7. The two series of one pair distinct; hydrothecse 



cylindrical, long S. Allmani. 



The two series of one pair not distinct ; hydrothecfe 



not cylindrical, more or less quadrate 8. fusca. 



8. Hydrothecse arranged in three series 8. triserialia. 



Hydrothecse forming more than three series; colonies 



of the form of Thujaria thuja 9. 



9. Hydrothecse entirely immersed in the substance of 



the branch ; oritice with a smooth margin 10. 



Hydrothecse projecting at their upper part ; orifice 

 with two teeth .... 11. 



10. Three branches forming a system ; series of hydro- 



thecse ten (at least) 8. decemsenalis. 



Six branches forming a system ; series of hydro- 

 thecse six or seven 8. thvja. 



11. Four branches forming a system; series eight or 



nine ? 8. hidentata. 



Six branches forming a system; series eight or nine 8. ochotensis. 



I now pass to tlie description of a new species of Sertularia^ 

 and of a very interesting Sertularella, both from the North 

 Pacific Ocean. 



Sertularia compi-essa, sp. n. (PI. XVII. figs. 17-19.) 



Trophosonie. Hydrorhiza in the form of stolons. Hydro- 

 caulus short, erect, not angular, rather rigid, divided into irre- 

 gular internodes, only giving off very few ramifications. 

 Branches arranged alternately and regularly on two sides of 

 the principal stem, straight, also divided into irregular inter- 

 nodes. Hydrothecse arranged alternately, subopposite, one to 

 three pairs in each internode, the base inflated and rounded, 

 the upper half strongly compressed in a plane vertical to the 



