154 BRITISH SEA-WEEDS. 



impossible to detach a perfect specimen. In structure 

 and fructification there is a close affinity between this 

 genus and Melobesia ; but the coating of carbonate of 

 lirne, which distinguishes the latter genus, is altogether 

 absent from Hildenbi^andtia. 



Genus LX. PETROCELIS. 



Frond spreading, in a leathery skin, over rocks, composed 

 of simple, jointed fibres set in gelatine. Spores unknown ; 

 tetraspores cruciate, formed of the middle joint of the fibres, 

 which compose the frond. — Petkocelis, from the Greek 

 petros, a rock, and cTiele, a claw. 



This genus was formerly considered to be identical 

 with Cruoria. The separation is founded on the dif- 

 ferent arrangement of the tetraspores. 



Petrocelis omenta. The blood-red Petrocelis. 



Fronds spreading, in smooth, circular or irregular patches, 

 on rocks, gelatinous, composed of vertical filaments. 



This is another obscure species, destitute of external 

 beauty, and not likely to prove interesting to young 

 collectors. 



Genus LXI. CRUORIA. 



Frond expanded, composed of vertical, jointed fibres, which 

 are very closely packed, and surrounded by fluid gelatine. 

 Spores unknown ; tetraspores zonate, borne on transformed 

 side-branches of the fibres. — Ceuobia, from the Latin 

 cruor, blood. 



This genus, as now constituted, is comparatively new 

 to the British flora, but since it has been described nu- 

 merous specimens of both the species that it contains 



