SIPHONACEtE. 25 



Fronds solitary, erect, with a cuneiform stipes composed of several confluent articu- 

 lations and dividing at the summit into numerous branches, which soon again subdivide 

 in a di-poly-chotomous manner, all the branches lying in one plane, so as to form a 

 flabelliform frond. The articulations are all compressed, the middle ones more or less 

 cuneate ; the upper iisually tridentate or three-fingered, and frequently bearing articu- 

 lations from the summit of each lobe. Colour rather a bright green. Calcareous 

 incrustation thin. 



This is nearly related to H. incrassata, and perhaps only a variety. But the crust 

 is not so dense, and the distinctly three-lobed upper articulations are characteristic. 

 Ellis's figure correctly represents a small branch. 



Plate XLIV. C. Halimeda tridens ; the natural size. 



4. Halimeda tuna, Lamour. ; frond much branched, diiFuse, di-trichotomous ; articu- 

 lations flat, thin, very broad, roundish or somewhat reniform, mostly entire ; the middle 

 ones sometimes cuneate. Lamour. Pol. Flex. p. 309, t. W.fig. 8. Dne. Cor. p. 91. 

 Kiitz. Sp. Alg. p. 504. Corallina Tuna., Ell. and Sol. tab. 20, Jig. e. Hal. platy- 

 disca, Dne. ? p. 90. (Tab. XL. A.) 



Hab. Key West, W. H. H. Key Biscay ne. Prof. Tuomey. (v. v.) 



Root deeply descending. Stipes scarcely any, consisting of a single, cuneate or 

 flabelliform articulation, which is sometimes more than an inch across, from the upper 

 margin of which spring numerous other articulations, forming the bases of so many 

 irregularly dichotomous branches. The articulations vary much in form and size. 

 Their usual shape is roundish or sub-reniform, and they are from half to | inch broad, 

 quite flat, smooth, and thinner than in most species. They are joined together by very 

 much constricted nodes, and are usually broader than their length. In some of my 

 specimens, however, some of the medial articulations are narrow-cuneate or almost 

 clavate. There is less incrustation in this species than in most ; the colour is a bright 

 green, and the substance somewhat flexible. 



The original H. Tuna is a native of the Mediterranean ; and possibly the plant now 

 described may be referable to H. platydisca, Dne., but some of my Key West specimens 

 so closely resemble what I have received from the Mediterranean, that, habitat apart, I 

 cannot find a character to distinguish them. Others are certainly of larger size, witli 

 more discoid articulations. If, however, every slight variation in form and size is to 

 be made the foundation of a new species, and dignified with a special description and 

 name, the number of species to be established would only be limited by the num- 

 ber of specimens examined ; for scarcely two are to be found identical at all points. 



Plate XL. A. Halimeda Tuna, the natural size. 



