122 BRITISH DESMIDIEiE. 



The conjugated fronds are connected by the formation of a bag-hke recep- 

 tacle or cell, which is colourless and very thin, and therefore difficult of 

 detection. As this enlarges the fronds become more remote from each other, 

 their segments partially separate at the constriction on the inner side, and the 

 endochromes of both pass out and unite to form an orbicular body between 

 them. In this state it resembles the sporangium formed by some species of 

 Closterium. At first it is inclosed in an orbicular membrane much larger 

 than itself; but as it increases in density, fine hairs make their appearance 

 on the surface and gradually become stout spines ; the membrane then dis- 

 appears, and the sporangium acquires its perfect state covered Avith conspicuous 

 awl-shaped spines. At this stage the empty fronds seem scarcely connected 

 with the sporangium, except that they lie on opposite sides of it, have their 

 . openings towards it, accompany its movements, and always retain the same 

 relative position. 



The above description of the process applies, with occasional and slight 

 variations, to the conjugation of the greater number of species in the Des- 

 midiese whose sporangia have been noticed ; and I have detailed it at length 

 in this place, not only because this species has afforded more numerous 

 opportmiities for tracing the formation of its sporangium, but because it was 

 the first example I had witnessed of the spinous kind, and I devoted more 

 time to its examination than to any other. 



Either more than one species has been included under this name, or the 

 sporangia vary much in regard to their spines, which in the Penzance spe- 

 cimens are fewer and appear at first like minute tubercles. 



Staurastrum dejectum and a few other species form a distinct group, di- 

 stmguished by their smooth fronds, the peculiar inflated or mammillate form 

 of their angles, or rather lobes, in the end view, and by their terminal awn- 

 like spines. In some respects they have more resemblance to the two plants 

 placed in Arthrodesmus than to the other species of this genus. Should any 

 change be required, I would rather remove them to Arthrodesmus than unite 

 that genus with Staurastrum. 



Staurastrum dejectum is larger than S. cuspidatum, its spines are shorter, 

 and its segments are connected either without a band or by a very short one. 



Length of frond -^\^ of an inch ; breadth yiy ; breadth at constriction jtst '■> 

 length of awn ^g st '■> diameter of sporangium yi^ ; length of spine of spo- 

 rangium from 2-Jo^ to YaVo- 



Tab. XX. fig. 5. a, b, c. front \dews of frond ; d, e. end views ; /, y. con- 

 jugated fronds ; li, i, k, J, m. different states of sporangia. 



2. S. cuspidatum (Breb.) ; segments smooth, fusiform, connected by a 

 long narrow band ; awns parallel or converging, but straight ; end 

 view with inflated awned lobes. 



Binatella tricusjridafa, Brebisson, Ale/. Fal. p. 57. t. 8 (1835). 

 Staurastrum cuspidatum, Breb., Meneghini, Si/nop. Desmid. in Linufea 1840, 



p. 226. Brebisson, in fit. cum icone. 

 Phycastrum cuspidatum, Kutzing, Phj. Germ. p. 138 (1845). 



