400 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



A species not closely related to any of the foregoing ; exter- 

 nally characterised by the mostly spherical or broadly pyriform 

 segments. It appears to be a plant of deeper water than most 

 species of the genus. 



5. H. TUNA (Ell. and Sol.) Lamouroux, 1812, p. 186 ; ?Har- 

 vey, 1858. p. 25, PI. XL. A; P. B.-A., No. 1484. Plants not 

 usually over 10 cm. long, moderately calcified, branching in 

 one plane ; a few of the lower segments thick, the others thin, 

 about i mm., varying in form but mostly cuneate, not ribbed, 

 margin entire. Filaments of central strand fused in twos or 

 threes at the apex of each segment ; peripheral utricles 30-70 p 

 diam., adherent for one-twenty-fifth to one-tenth of their length, 

 rather easily separable ; utricles of subcortical layer 35-110/1* 

 diam.; sporangia globose to pyriform, 200-330 /u, diain., deep 

 green, borne on simple or forked pedicels, on margin or surface 

 of the segments. Fla., W. I. Europe, Asia. 



The group including //. Tuna, H. discoidea and H. scabra is 

 generally distributed in all warm waters, and distinguished by 

 thin, not heavily calcified segments. The roughened surface 

 distinguishes H. scabra, the slight calcification //. discoidea. 



6. H. DISCOIDEA Decaisne, 1842, p. 102 ; Howe, 1907, p. 

 495, PI. XXVI; P. B.-A., No. 1483. Plants reaching 15 cm. 

 in length and width, very slightly calcified, color bright green, 

 fading when dry ; branched in one plane ; segments deltoid to 

 elliptical with long axis transverse, up to 35 mm. broad, thin, 

 smooth, shining. Filaments of central strand fused in twos, 

 rarely threes, at the nodes ; peripheral utricles 40-85 p. diam., 

 often fusing, in contact with each other for one-fifth to two- 

 thirds their length, not easily separated. Utricles of subcortical 

 layer relatively large, bullate, 110-215 A 1 diam. Fla., W. I. 



Asia, Africa. 



Resembling H. Tuna, but less calcified and of thinner texture ; 

 the segments vary much in size, the largest are broader than 

 long, and of greater dimensions than in H. Tuna ; in cases where 

 the habit is not distinctive the shape and dimensions of the 

 utricles may be depended on to decide. 



7. H. OPUNTIA (L.) Lamouroux, 1812, p. 186; Harvey, 

 1858, p. 23, PI. XL.B; Vickers, 1908, p. 25, PI. XXXV; P. 

 B.-A., No. 123. Plants usually 10 cm. high, sometimes up to 

 25 cm.; more or less branched in various directions; segments 

 much calcified, very variable in shape ; discoid, cordate or tri- 

 lobed ; more or less plainly ribbed; up to 12X20 mm. Fila- 



