340 W. A. Setchell — Classification and Geographical 



digitate species and the presence or absence of mucilage glands in 

 the stipe, which Ktitzing^ has used to separate Hafgygia from 

 Laminaria, yet in general structure they are all similar and agree 

 perfectly in the most important characters, viz : the perfectly un- 

 modified blade and transition-place. The features just mentioned, 

 although remarkable and important, are to be looked upon as the 

 variations among individual species or small groups of species. 



Subtribe 2. Agarese. — The genera included under this subtribe 

 are Agarum, Thalassiophyllum, Costaria, Cymathmre, and perhaps 

 also Arthrothamnus. 



Agarum (Bory), P.&R. — The genus Agarum, although occurring 

 both in the northern Atlantic and the northern Pacific, probably con- 

 tains but a single species. The individuals however vary greatly 

 in size, outline of the frond, the size and shape of the perforations, 

 etc.,^ but it is hard to find fixed characters to separate them.' 



In Agarum Turneri, P.&R., we find a stipe of very variable 

 length (from an inch or two up to a foot or more) which bears at 

 its tip the heart-shaped blade. Where the blade arises from the 

 transition-place, which appears here to form a portion of the stipe, 

 it unrolls itself from two small conical scrolls, each of which is 

 involute upon the stipe into a sort of spiral. These scrolls at the 

 base of the blade are present even in young plants of about 2 inches 

 in length and probably also in still smaller ones. J. G. Agardh 

 noticed these scrolls, for he says on page 3 of " De Laminarieis;" 

 "Sunt in his frondes una pagina plus minus ab altera dissimiles; 

 ssepe basi convolutse, gerunt unam internam, alteram externam 

 faciem;" and a few lines farther on: " In Agaro est stipes revera ad 

 ortum frondis canaliculatus et frondis lamina, secus margines stipitis 

 decurrens, involuta." This is the only mention of these scrolls 

 known to the writer. 



The stipe is continued up through the blade as a sort of broad 

 midrib. The blade is perforated with holes of various sizes, the 

 development of which has been described by Humphrey.* 



Thalassiophyllum , P.&R. — Thalassiophyllum Clathrus, P.&R., 

 the only representative of this curious genus, is a resident of the 



' Phyc. Gen., p. 346, Species Alg., p. 577. also Areschoug, *bs. Phyc, Pt. 4, p. 1. 



2 Cf. P. & K, 111. Alg., PI. 20-23, 1840. 



3 Cf. Farlow, New Engl. Alg., pp. 96-97, 1881, as to variability of the Xew England 

 species. '* Proc. Am. Acad., vol. Ixxiii, pp. 201-203. 1886. 



