242 DICTYURUS. PAET n. 



often elegantly branched threads, while such genera as 

 Amansia and Odonthalia have instead a flat and pinna- 

 tifid frond. The latter, which has a very conspicuous 

 cellular reticulation, is a genus of high latitudes, but is 

 common on some parts of the Scotch and North American 

 coasts. The British seas are rich in many genera of this 

 order, and analogous forms occur in the southern hemi- 

 sphere, where there are at least twenty-three genera. 

 Many are remarkable for their singularity of structure : 

 the Claudea for example, which is one of the most elegant 

 of the Algae, has a cancellated frond and is the ornament 

 of warm seas ; the Amansia and Leveillea which are dis- 

 tinguished by the beautiful reticulation of their fronds 

 caused by large hexagonal cells ; and the Dictyurus, 

 in which the net forms a spiral web round the prin- 

 cipal stem. Tig. 27 shows a por- 

 tion of the network of Dictyurus 

 purpurascens magnified. All the 

 genera of this order possess free 

 areolate hollow conceptacles per- 

 forated above, and containing 

 nuclei, from the base of which 

 short tufts of threads arise, each 

 bearing a large obovate spore at 

 its apex. The tetraspores are ar- 

 ranged in series either within the 



O 



frond, or in distinct pod-like re- 

 ... Dtcty. purree. ceptadflB called stichidia. Fig. 



28 shows the Polyzoma cunei- 



folia with its tetraspores arranged in rows in their 

 pod -like stichidia, together with the areolated concep- 

 tacle and spores, all highly magnified. The antheridia 

 differ in form in the different genera. In the Dasya 

 they assume that of pods full of cells, in which the 

 motile particles are generated ; in the Rytiphlsea tinc- 

 toria the antheridia resemble those of the Dasya except 



