268 Siphonodadiales 



of Rhizoclonium, and the systematic account of the genus given by Stockmeyer 

 in 1890 is a thoroughly reliable one. 



The genera are : Rhizoclonium Kiitzing, 1843 ; Spongomorpha (Kiitzing, 1843) Wille, 

 1909 ; Chxtonella Schmidle, 1901. 



Species of Rhizoclonium occur abundantly in marine and brackish situations, the two 

 forms, f. tortuosum (Ku'tz.) Stockm. and f. riparium (Harv.) Stockm., of R. hieroglyphicum 

 being characteristic of the mud of salt-marshes in north-west Europe ; and several other 

 forms of this species are also common in fresh water. A few species are subaerial in 

 habit, occurring on damp soil, and some occur in the warm streams of active volcanic areas. 

 Spongomorpha is exclusively a marine genus, and Chsetonella is a small freshwater Alga 

 which is sometimes epiphytic in the mucous coats of larger Algae. Great care is required 

 in the discrimination of species of Rhizoclonium owing to the variability of the vegetative 

 characters (Brand, '98). In R. profundum there are no true rhizoidal branches, and the 

 commonest form of R. hieroglyphicum is entirely destitute of lateral branches. 



Family Dasycladacese. 



In this family are included a number of Green Algas in which the thallus 

 consists of a much elongated axial segment (generally cylindrical and firmly 

 attached to the substratum by branched unseptate rhizoids) which bears in 

 its more apical portion numerous acropetal whorls of branches. The latter 

 are of limited growth, simple or again branched, and segmented. All the 

 segments are ccenocytes. In some forms the thallus is encrusted with 

 calcium carbonate. The chloroplasts are small and discoidal, as in so many 

 of the Siphonodadiales, and each one is provided with a pyrenoid. 



Zoogonidia have not been observed in any member of this family, but all 

 the segments of the lateral branches, or certain segments only, may either 

 become gametangia in which numerous isoplanogametes arise, or produce 

 small cosnocysts of the nature of aplanospores. The latter usually germinate 

 directly, but may become gametangia. 



Nearly all the members of the family live in tropical or subtropical seas. 



There are quite a number of fossil forms of the Dasycladacese from Cretaceous, Eocene 

 and Miocene deposits, and Diplopora and Oyroporella are known from the Trias. For 

 information on some of these fossil Algee consult Solms-Laubach ('91), Seward ('98), and 

 Garwood ('13). Of the Cretaceous forms, Neomeris cretacea Steinmanu (1899), Munieria 

 baconica v. Hanklen (1883), Diplopordla Muhlbergii Lorenz (1902), and Triploporella 

 Fraasi Steinmann (1880) are the most important. The latter is intermediate between the 

 Dasycladete and the Bornetelleae, and might well form, as suggested by Oltmanns, the type 

 of another sub-family, the Triploporellese. In this fossil form the lateral branches of the 

 first order swell out and flatten themselves against each other to form a compact strobilus- 

 like structure. The branches of higher orders were apparently rather delicate and deciduous. 



Of the Eocene Dasycladete, Decaisnella, Haploporella, Dactyloporella, Dactylopora, 

 Uteria and others have been described ; and of the Acetabulariese, Acicularia Andrussowii 

 has been described from the Miocene rocks south of Sevastopol, and A. miocenica from the 

 Miocene of the Vienna district. A. pavantina was described by D'Archiac from the 

 Eocene sands of the Paris basin. 



