ceramiacejE. 229 



Callithamnion tripinnatum. The thrice-pinnate 

 Callithamnion. 



Fronds growing in tufts, from one to two inches high, 

 membranaceous, fan-shaped, about thrice pinnate ; plumules 

 pinnate below, bipinnate above ; the pinna? short and sim- 

 ple, becoming gradually longer and branched to the middle 

 of the plumule, and thence gradually shorter to the tip ; 

 pinnae and pinnules alternate, the former naked for the first 

 half of their length, with the exception of the joint next 

 the stem, which usually bears a short pinnule ; joints of the 

 stem and branches about thrice as long as broad. Favellse 

 unknown ; tetraspores tripartite, on the upper sides of the 

 basal and ultimate pinnules. 



This exquisite little plant is unfortunately very rare. 

 It grows on rocks at extreme low- water mark, is annual, 

 and in perfection at the end of spring. Its distinctive 

 character is the solitary, generally fertile pinnule, that is 

 borne on the first joint of each pinna of the plumule. 



Callithamnion affine. The allied Callithamnion. 



Fronds growing in tufts, from two to three inches high, 

 feather-like, much branched ; stem opake, veined ; branches 

 long, with a roundish outline ; plumules alternate, short, 

 simple, awl-shaped ; articulations of the branches three or 

 four times, of the pinnse about once as long as broad. 

 Favellse in pairs, near the tip of the plumules ; tetraspores 

 tripartite, solitary, near the base of the pinnules. 



The title of this plant to specific rank is of the most 

 doubtful nature. It partakes of the characters of the 

 normal states of three or four species, and is scarcely 

 separable, with any degree of certainty, from some of 

 their intermediate forms. Both Harvey and Agardh 

 have hitherto retained it, and therefore I insert it here. 



