CERAMIACE^. 231 



summer. There is a close resemblance between it and 

 C. thuyoideum, they both bear their tetraspores at the 

 tips of the pinnules, and in this character differ from 

 all the allied species. 



Callithainnion corymbosum. The corymbose 

 Callithamnion. 



Eronds solitary, or growing in tufts, distinctly jointed 

 throughout, much branched ; stem and branches as thick as 

 a bristle below, extremely slender above ; plumules very 

 fine, flaccid ; pinnsB opposite, alternate or secund (on one 

 side only), the lesser and ultimate divisions being fortedly 

 divided, and level-topped ; articulations of the branches 

 sometimes veined, six or eight times as long as broad, of the 

 branchlets shorter. Pavellse in pairs, on the rachis of the 

 plumules ; tetraspores tripartite, solitary, near the axils of 

 the forked pinnae of the plumules. 



This species is very variable, and was formerly divided 

 into two : the short-jointed, pinnately-branched, robust 

 form being called C. versicolor. Dr. Harvey pointed 

 out the error of this arrangement ; and any one who will 

 take the trouble to examine an extended series of spe- 

 cimens will be convinced, I think, that the two forms 

 belong to the same species. C. corymbosum grows on 

 Zostera, sea-weeds, or rocks, near low-water mark. It 

 is annual, and in perfection in summer. 



Callithamnion byssoideum. The byssus-Iike 

 Callithamnion. 



Eronds growing in dense tufts, from one to three inches 

 high, extremely fine, flaccid, tender, jointed nearly to the 

 base, much branched ; lower branches irregularly divided, 



