(2) 



or receptacles. Receptacles at first ovate or lanceolate, at length linear 

 oblong, loosely cellular, containing under the siu-face numerous spherical 

 conceptacles, communicating with the surface by a minute pore, and 

 having attached to their inner surface numerous oblong spores, rounded 

 at the ends, somewhat contracted in the middle, and mixed with branched 

 articulated filaments, bearing bright orange-coloured " antheridia." Struc- 

 ture : substance coriaceous, more or less woody in the older parts, scarcely 

 adhering to pajDcr. Colour, when young, a greenish olive, more or less 

 brownish or olive-brown when old, blackish brown when dry. 



This fine species is readily distinguished from every other British Alga 

 by the curious stmcture of its air-vessels, which are divided transversely 

 by extremely thin membranous diaphragms, and these are curiously sup- 

 ported by very slender filaments, stretched from diaphragm to diaphragm, 

 like the chords of a hai-j^. When the specimens are fresh no constriction 

 is visible externally, corresponding to these diaphragms ; but as the 

 air-vessels dry the diaphragm contracts, and a corresponding external 

 constriction becomes very apparent. In old plants these constrictions 

 are also frequently more or less visible. The air-vessels are frequently 

 produced at the expense of a branch, and as the plant advances in its 

 growth, often drop off" or are removed by abrasion ; hence the irregularity 

 of the older pinnse. 



The genus Halidrys is neither numerous in species, nor widely dis- 

 tributed, being confined perhajDS to the North Atlantic and its arms ; 

 but Professor Harvey mentions a very closely allied species, a native of 

 the west coast of North America. The present species is very common 

 on the British shores, both on the east and west coasts, between tides, 

 but never exposed to be left dry, being in pools where it can always 

 be covered by the tide. 



In shallow pools it is often very stunted and dwarfish in all its parts, 

 and in this state is very possibly the variety /3 of authors ; but we have 

 never seen the plant present so much variation as to deserve to be 

 distinguished even as a variety, indeed the plant is remarkably constant 

 to its character in every part. 



EXPLANATION OF PLATE CXXX. 



Fig. 1. — Halidrys siliquosa, natural size. 

 2. — Section of vesicle. 

 3. — Section of receptacle. 

 4. — Semisection of conceptacle. 

 5. — A spore. All magnified. 



