124 THE JOURNAL OF BOTANY 



herbarium, but I am not aware that it was ever pubHshed by 

 Mitten as a British plant. The specimen has been kindly sent to 

 me by Mrs. Britten. There are two sheets. One has been 

 labelled at New York, " Found with Brachrjthecium trachypodium 

 (Brid.)." It is a good fruiting specimen of trachyj^odium, with 

 Mitten's sketches attached, no locality or date, the only data given 

 being " Mr. Black" in Mitten's hand. It is pretty obvious that 

 this is a Continental specimen placed side by side with the British 

 plant for comparison. The other consists of a small packet, 

 labelled in Mitten's hand " Hypnum tr achy Imodium '^. Encalypta 

 rhahdocariM,-'' Ben Lawers, Hb. Hooker." It is accompanied by 

 a drawing of a leaf and basal cells, which might very well 

 represent B. trachypodium. The specimen itself is mainly the 

 Encalypta, with one or two stems of a Brachythecium not unlike 

 some of the less well characterized forms of B. trachypodiuvi, and 

 with leaves that certainly show decided though weak striation — 

 one of the principal characters separating B. trachypodium from 

 B. velutinum. For all this I should consider it to belong to 

 B. velutinum rather than to B. trachypodmm. The latter species 

 is usually distinguished by its more robust, more rigid habit, with 

 rather stout obtuse branches; the Ben Lawers plant is more silky, 

 with rather slender, tapering branches. In habit and leaf form 

 the Ben Lawers specimen is exactly in agreement with certain 

 forms of B. velutinum, notably var. pralongum B. & S. (e. g. Husnot, 

 Musci Gall. No. 741), to which var. I should refer it. I do not 

 think the slight striation of the leaves, unusual as it is in B. velu- 

 tinum, can be held to outweigh the other characters, and I should 

 certainly consider it very unsafe to base a British record of 

 B. trachypodium on this plant. The specimens in question are 

 now placed, at Mrs. Britten's request, in the Kew Herbarium. 



INDEX SPECIES IN A FLOEA. 

 By the Eev. E. Adrian Woodruffe-Peacock, F.L.S. 



What is an index species ? A plant that points out the in- 

 coming of a new combination of circumstances, or the existence 

 of such circumstances when hidden from view. 



A party of botanists were working a marsh dyke with its glory 

 of Lythrum, Utricularia, Sparganium, Juncus, and Carices. 

 Suddenly they came on Stachys palustris, intruding, as it were, 

 amongst the true marsh species. The banks of the dyke were 

 still firm peat, of the Sparganium, Juncus, and Carices formation, 

 but the bed of the dyke had reached the Oxford Clay lying below 

 at the spot, and this permitted S. palustris to survive and flourish. 

 There was nothing very wonderful in being able to point out the 

 junction spot in a second and this plant as its index species ; the 

 wonder should be that such things are ever passed over, but it 



• This has been written over " Btreptocarpa,^' but the specimen is E. 



rhabdocarpa. 



