8o 



SCIENCE- GOSSIP. 



rocking in a dish or soup-plate, as described in 

 previous papers of this series. In this particular 

 gathering Micrasterias jenneri was extremely 

 abundant, but a few specimens of M. oscitans as 

 well as of the XantMdium occur on most of the 

 spread slides. Some of the larger species of 

 Micrasterias are still more graceful, notably M. 

 radiosa, M.furcata, and M. denticvlata, of which 

 the two first occur on an old slide of Joshua's from 

 Capel Curig, mounted twenty years ago, that latety 

 I succeeded in restoring. Seventeen species of this 

 genus are figured in " Cooke." 



Of the genus Euastrum, the species represented 

 in fig. 4, namely, E. verrucosus, is a pretty 

 example with its surface covered with dots, but it 

 can hardly be called a typical species, as the 

 majority have the frond about twice as long as 

 broad. Figs. 5 and 6 show the outlines, traced with 

 a camera lucida, of the two species, E. didelta and 

 E. cuneatuvi, collected in Sphagnum near Llyn 



Pig. 3. fig. 4. 



Micrasterias oscitans. Fig. 3. M. jenneri. 



Fig. 4. Euastrum verrucosum. 



Idwal, which appears to be a favourite habitat of 

 the genus. Excluding varieties, Dr. Cooke enu- 

 merates twenty-eight species, mostly characterised 

 by the pyramidal shape of their segments with 

 sinuous outlines. A pretty form is E. oblongum, 

 with its front of oval shape and either segment 

 deeply 5-lobed, the central lobe being notched at 

 the apex, as in other species of the genus. 



Cosmocladium is a small genus containing two 

 very local species with the individuals united by 

 dichotomously branched filaments. 



The next genus Cosmarivm contains at least 

 ninety-seven British species, some of them among 

 the commonest of desmids. A pretty form, C. 

 ralfsii, with smooth cells is represented by a 

 camera lucida outline in fig. 7; it is a large 

 species, frequent in Sphagnum, washings from 

 Llyn Idwal, and of a beautiful green tint when 

 fresh. Another form is seen in vol. vi. p. 257, fig. 1, E. 

 Most of the species of Cosmarivm have the cells 

 granulated or dotted, and the segments reniform 

 and broader than long. Some species occur' com- 

 monly in almost any ditch or pool, but often 

 associated with so many other forms of pond life 

 that it would be difficult to clean and mount them. 

 In the Easter vacation of 1891 I found in some of 

 the pools on Eeigate Heath, Surrey, green masses 

 which proved to be made up entirely of a small 

 species of Cosmarivm. 



A camera lucida outline of a desmid which I 

 refer to, the genus Calocylindrus, is shown in fig.. 

 8, from which it will be seen that the species of 

 this genus have their segments generally more 

 elongated than those of Co&warium, and with little- 

 constriction between them ; in some species there is 

 not any such narrowing. The specimens from which 

 the outline was sketched formed green masses in. 



Fig. 5. Eiiastrum didelta. 

 Fig. 6. E. cuneatum. 



Fig. 7. Fig. 8., 



Fig. 7. Cosmarium ralfsii. 

 Fig. 8. Calocylindrus. 



some roadside pools just after a shower of rain, and 

 looked as if there was some gelatinous connection 

 holding the individuals together as the masses 

 floated about in the water. 



Fig. 9, c, shows a characteristic form of the genus- 

 XantMdium occurring frequently in the Sphagnum 

 washings from the Nant Ffrancon valley. They are 

 characterised by the spines upon them, the seg- 

 ments are obtuse, with a deep constriction between 

 them, and ten species are described in " Cooke." 

 The name Xanthidia is also given to the curious 

 fossil bodies met with in thin chippings of flint,. 



FlG. 9. «, a, a, M. jenneri ; b, b, M. oscitans ; 

 c, Xanthidium. 



from their resemblance to the spores of desmids. 

 A closely allied genus Aithrodesmus, likewise with- 

 spiny segments, is characterised by the fact that 

 while the species of XantMdium have a circular pro- 

 jection on both faces near the centre, this is absent 

 from Arthrodesmus. 



The last of the desmid genera, Staurastrum, with 

 over ninety species, exclusive of varieties, is no 

 doubt a puzzling genus and most of the species 

 are too small to make good drawings or photo- 

 graphs. One small species may be seen in end. 



