390 



.lOHS. IJOYi; l'liTi:HSEN 



Originally Ihcso (liHcionl forms wcmc described :is variclies or t-vcii 

 as distinct species, but (>levc reduces Iheni all to formæ. Syn. I. p 129). 

 Transition forms occur betwcen various of tlie forms but 1 am not sure 

 wbetlier il is tbe rase witli all of tliem Tlic systemalic rank of tliese 

 forms can ])robably only be establishcd by ciilture experiments. Kor 

 ecological purposes it seems to me convenient to keej) tliem distinct for 

 the present. \\'hetlier we call them species, varieties. or foiMns is of no 

 great consecjuence. .\s I have liail fairly good material under observa- 

 tion of this s|)ecies. I shall here attempt lo give a general view of its 

 various forms as far as iny observntions go. 



A. Valve without undulated edges 



I. Valve perfectly symmetricai. elongate-elli])lic. slightly 

 produceil rounded apices, strialion closer al apices 

 than in middle 



II. Valve + a.symmetrical 



/\ Valve shorler or longer linear 



1. Apices protractæ 



2. — capitatæ 



/\,/\ Valve elliplic-lanceolate 

 * Length more than \3 n 



a. Apices produeed, obtuse 



b. Apices not produeed 



1. Valve lanceolale. abt. 3 times as long as 

 broad, ajMces acute 



2. Valve lanceolale, swoUen in middle. 

 apices obtuse 



3. Valve abt. twice as long as broad. apices 

 rounded. distinct apical area 



f. Wulf/ii. 



f. p rod u c la. 

 I. nentricosa. 



f. Cohnii 



** Length less than 13 // 



B. Valve with undulated edges, 



f. Goppcrliana. 



f. rhomboidea. 



f. elliplica. 

 f. miniinn. 



f. iindiildla. 

 f. l'i'iiiKtna. 

 f. Li'niimcn. 

 f. (ininqiicnodis. 



( niixdis-?) 

 f. hinodis IIiisl. 

 I. rhoinbica. 



Of the forms with undulated valve edges I have only observed 

 f. qniiupieiwdis. I cannot therefore speak of them liom personal ins])ec- 

 tion, nor can I say anything about the forms described by PUtiifttir (10]4, 

 p. 1 1 2 , viz. \. oimlis, w suhhexayona, v. suhrirculdris. 



NB. Forms showing a coarse struclure like thai of A'. m. Cohnii 

 were observed in sonie samples from Vestmannaeyjar. They approach 

 N. mulico/)si.s West. I have seen no pronounced f. Gopperliana V. Heurck 

 Syn. PI. \, lig. 1«. 18b. Van Heurck, Types Xo. 114) in Iceland. but 



