114 GELIDIACE^. v. 



o-enera of the Order this is obviously the case, the conceptacle arising from a 

 sweUino- in the middle of a branchlet wliich is continued beyond the swollen part. 

 In such a case the portion of the substance which constituted the axis of the frond 

 below and above the conceptacle becomes the placenta while passing through the 

 cavity. In Eucheuma, where the whole branchlet is encysted, this structure is less 

 apparent, but the filaments which connect the suspended placenta of that genus 

 Avith the walls of the conceptacle show that the axis does not terminate at the 

 placenta, these filaments being continuations of it. In Fterocladia an anomalous 

 structure arises from a different cause, namely from the conceptacular cavity being 

 formed not at both sides of the axis as in Gelidlum, l^ut at one side only. The 

 result is, that the portion of the axis that bears spore-threads, instead of passing 

 through the centre of the cavity, occupies its inner face as a parietal placenta. 



Takino- this view of the placentation, the connection between the conceptacles of 

 Gelidlum and of Hypnea may be easily understood. In Gelidium, as has been 

 already said, the axis is in excess, while in Hiipnea it is deficient ; and the placentte 

 being a development of the axis, we should expect a corresponding relation in the 

 placentation of these genera. And such is the case. In Gelidium a dense, fibro- 

 cellular placenta, corresponding with the dense axis, divides the conceptacle 

 through the middle of wdiich it runs into two loculi, and bears spore-threads 

 on both surfaces. In Hypnea the spore-threads spring from slender, cobweb-like 

 filaments running through the cavity. The structure of Sol'mia is intermediate, 

 there being in the conceptacle of that genus a fibrocellular placenta suspended in 

 the cavity by filaments connected with the surrounding walls, and bearing spore- 

 threads over the whole outward surface. This genus and Eucheuma, which has an 

 analogous structure, appear to me to connect Hypnea so far with Gelidium as to 

 render it advisalile to place them in the same group. And possibly Chcetangium 

 also, when carefully compared with Fterocladia, may be found, notwithstanding its 

 remarkable characters, to have a close relationship. 



The Gelidiacete, as here defined, are remarkable for their beauty, the sportive 

 forms Avhich many species assume, and the wide geographical range of some. Geli- 

 dium cartilagineum has been noticed for the brilliant variety of the colours which it 

 puts on under the bleaching action of the sun and air, before it fades altogether ; 

 but Hypnea musciformis, Eucheuma isiforme, and sevc ra' others are equally brilliant 

 under similar circumstances. Gelidium corneum, a most variable plant, is found 

 throughout most pai'ts of the Atlantic and Pacific Oceans, if we except high north- 

 ern and southern latitudes. It is however both rare and of small size on the eastern 

 coast of North Amei'ica, though on the Pacific coast it is apparently common and 

 attains large dimensions. Solieria chordalis, so abundant on the American shore, is 

 of rare occurrence and rather local in Europe. Hypnea musciformis abounds in all 

 tropical and subtropical seas. Suhria and Ftilophora are only known at the Cape of 

 Good Hope, and Fterocladia is one of the most abundant and characteristic of New 

 Zealand Alga3. Caulacanthus, associated wath Hypnea by Professor Agardh, is of 

 doubtful aflinity, its conceptacles being unknown ; and I rather incline, with 

 Kiitzing, to place it near Endocladia. 



