ARCHER AND DIXON, ON DESMIDIACE.E. 79 



Length of frond, ^i^ to t^j breadth of frond, ^fa; 

 breadth at constriction, —i^g of an inch. 



Plate VII. Fig. 3, front view ; fig. 4, end view. 



This, although not a complex form, owing to its brilliant 

 and beautifully radiately- disposed endochrorne (in front view 

 almost in fillets), is an extremely pretty species. In end view 

 its non-inflated mucronate angles and series of papilla3 dis- 

 tinguish it, I think, from every other Staurastrum. In 

 front view it somewhat resembles S. asperum, Breb., a, but 

 the minute spines on the outer margin, in that species, are 

 usually emarginate or cleft at the ends, or dilated, and the 

 segments are not mucronate at each angle, nor is the endo- 

 chrorne radiately disposed. In the form in question, the 

 convex sides, mucronate angles in end view, and granules 

 not scattered, distinguish it from S. punctulatum. I do not 

 think I need contrast it with any other species, and I believe 

 that both the foregoing forms have only to be seen, when 

 their perfect distinctness would be at once apparent. 



New Genus and Species in the Desmidiace^b. 



I beg leave to submit to the notice of your Society the 

 following account of a form of Desmid, which I have lately 

 met with in this neighbourhood, and which I believe has not 

 hitherto been described. The frond, as represented in Plate 

 VII, figs. 5 — 7, is simple, compressed, with a deep and acute 

 gaping constriction between its segments, which are tln-ee- 

 lobed, the line separating the extreme from the basal lobes 

 being parallel to the line of separation of the segments. It 

 has no inflation on its surface, but exhibits on its margin a 

 few mucronate spines. This form appears to me to be gene- 

 rically distinct from both Micrasterias and Euastrum : from 

 the former in the direction of the separation of its lobes, from 

 the latter in the absence of inflations. In these characteristics 

 it agrees with Micrasterias oscitans and M . pinnatifida, Ralfs, 

 as well as with the form Holocystis oscitans, Hassall, de- 

 scribed by Dr. Hassall, and referred to by Mr. Ralfs (' Bri- 

 tish Desmidiese/ pp. 69 — 77), and it is worth considering 

 whether these forms should not be all grouped together in a 

 new genus. Before proceeding, however, to give the com- 

 plete description of this proposed genus and the three species 

 which it would contain, I beg leave to offer a few remarks on 

 the different manner in which the segments are divided in 

 those Desmids which have lobed or divided segments, and 



