3G0 WILLIAM ARCHER. 



substance/' Obliquely quadripartite is thus tlic great feature 

 of this form. 



Not less problematic, on all points, is the third form (figs. 

 11, 12) I would draw attention to, one which, so far as I can 

 learn, has not hitherto met observation •} it is clearly, how- 

 ever, one of kindred nature to the foregoing. The most 

 ready manner to convey an idea of the figure of this also very 

 minute form is, to say it resembles that of an Arthrodesmus ; 

 in other words, it is compressed, quadrangular, divided by 

 two opposite, deep, wide, triangular notches into two broad 

 cuneate segments, their lower (or inner) sides slightly convex, 

 and the outer angle subacute, and very minutely apiculate, 

 and the upper (or outer) sides very slightly concave at the 

 middle, somewhat raised towards the angles,^ the contents 

 pale seruginous colour, and homogeneous looking. This form 

 occurs of different sizes, some of them being twice the linear 

 measurements of others, but, as mentioned, even the largest is 

 exceedingly minute. Though thus arthrodesmoid in figure, 

 apart from the colour and a general aspect of contents, the 

 greatly differing sizes occurring in one and the same gather- 

 ing Avould prevent this being taken at all for any desmid ; 

 neither in this nor the following form is any gelatinous 

 envelope apparent. A certain amount of a kind of move- 

 ment shown by this form is, perhaps, not at all special, yet 



' During the interval that the above paper was in the hands of the 

 printer, I enjoyed the great gratification and valued privilege of making 

 the personal acquaintance of Dr. Veit B. Wittrock, of the University of 

 Upsala, on the occasion of a (too brief) visit of his to tliis country — an 

 occasion of which I must ever retain a lively and pleasing recollection. We 

 spoke, iiiter alia, of these forms, the subject arising from his having shown 

 me a sketch of a minute form found by him in Sweden, which I at once 

 recognised as most probably identical with that above recorded as Treirapedia 

 Reinschiaiia. I had fortunately, ere long, an opportunity to show Dr. 

 "VVittrock, a fresh example not only of this, but likewise of the other form 

 DOW here named Tetrapedia setigera (we were at the moment in the far 

 West, in the middle of Connemara's wilds), and 1 hope that skilled and 

 accomplished observer may not consider it too great a liberty on my part to 

 mention tliat, whilst he identified the former as occurring in his country, he 

 likewise concurred in my view above expressed in regard to both, as to their 

 nature and affinity. 



' I regret that the figures in the plate (figs. 11 and 12) do not sufficiently 

 accurately indicate the characteristic sought to be described by the words 

 " somewhat raised towards the angles ;" the figures show the upper margin 

 of the segment too uniformly concave, whereas each lateral portion or lobe 

 of the segment presents rather a mammillate outline, that is the upper 

 margin (as well as the lower) slightly convex towards tiie angles, but con- 

 cave at the middle. It will be, of course, understood that the two figures 

 (11 and 12) each show an individual, one with the "segments" lying 

 horizontally, the other vertically, the deep sinus being that making the 

 bilaterality, or arthrodesmoid contour, 



