Udote% 



filaments diverge to form a mop-like mass of freely branching threads which, 

 as a rule, are much less incrusted than the stalk. In some forms the apical 

 mop of filaments is small and compact, with well calcified filaments, whereas 

 in others it consists of long, straggling, and but slightly calcified filaments. 

 In the former it is rarely more than 2 cms. in diameter, but in the latter it 

 frequently attains a diameter of 10 15 cms. 



The curious type Rliipocephalus has a general resemblance to Penicillus 

 in habit and structure. Its stalk is round, and the capitulum, unlike that of 

 Penicillus, consists of a cone of many small cuneate flabelliform fronds. 

 These are 5 20 mm. in length, monostromatic, ascending and imbricating. 

 They are normally arranged in subverticils, and in R. pltoenix the Habelliform 

 fronds of the same whorl are usually laterally connate into collars which 

 more or less completely encircle the main axis. 







The finely divided rhizoids with which all the Udotea3 are furnished, enable them to 

 grow on a loose sandy or muddy bottom, the plants being fixed by the knitting together 

 of the loose material in such a way that the latter replaces a fixed substratum. Upon a 

 loose bottom of this kind in the West Indies Borgesen ('11) states that Penicilhis capitatus, 

 varieties of Halimeda incrassata, and Udotea flabellata, together with a few other species, 

 form a very luxuriant vegetation. Sometimes they are mixed, but more often they occur 

 in nearly pure unmixed societies. Thus, Penicillus capitatus grows so densely, head 

 against head, that the ground is scarcely visible. The forms from the open sea, which 

 occur down to 16 fathoms, are more firmly and compactly built than those from the 

 lagoons. Halimeda Opuntia grows vigorously in the lagoons, where it often forms low 

 banks or reefs, mud and organic detritus accumulating among its branches. In this way 

 it may form mound -like banks which die in the centre and send out new shoots at the 

 periphery. 



Some species of Udotea, such as U. ci/athiformis, U. spinulosa, etc., are only found in 

 deep water down to a depth of about 40 metres (22 fathoms). 



The Udoteee although represented in more northerly seas (Pe/t trill its, Udotea and 

 Halimeda have each one species in the Mediterranean) are mostly tropical, and in the 

 West Indies they are particularly abundant. Udotea and Halimeda are also abundant in 

 the Eastern hemisphere. 



The genera are : Tydemania Weber van Bosse, 1901 ; Penicilhis Lamarck, 1813 ; 

 Rhipocephalus Kutzing, 1843 ; Udotea Lamouroux, 1812 ; Halimeda Lainouroux, 1812. 



Owing to the calcification of the thallus the Udoteas unquestionably lend themselves 

 to preservation as fossils. Sollas has shown that the joints of species of Halimeda form 

 no inconsiderable part of the material which goes to constitute some of the Pacific 

 coral-reefs, and Borgesen points out that in the West Indies these joints often form an 

 appreciable portion of the coarse particles of the calcareous sea-beach. There is every 

 reason to believe that similar siphonaceous Algie have played some part in the formation 

 of the coral reefs and calcareous shore-deposits of previous geological periods ; in fact, 

 Fuchs has described a species of Halimeda (//. Saportse) from the Eocene. A Tertiary 

 Alga from the Paris basin very closely akin to Penicillus has been recognized in a fossil 

 state by Munier-Chalmers, and the Eocene fossil described by Lamarck as Ovulites is 

 very likely a siphonaceous Alga belonging to the Udotea). Sphcerocodium Bornemanm 

 described by Rothpletz as a fossil member of the Codiacere is exceedingly doubtful. For 

 further information on these fossils consult Seward ('98). 



