KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 27. N:() 3. 53 



The group most nearly connected with the Nav. Lyratie is the Nav. punctaffe, both being 

 united by numerous transitional forms. In some cases it is diHicnlt to decide whether a form 

 belongs to the Nav. ))unctat{e or the Nav. lyrata-. Such forms are for instance N. trun.sfu;ia and 

 N. carinifera among the Punctatti; and N. Srhuarsckmidtii among the Lyratae. 



On the other hand the Nav. lyratcB offer some resemblance to the genus Diplouris, especially 

 in I). Iiyuiina, D. Httdsonis and some other' forms. The prolongations of the central nodule in 

 Diploneis correspond evidently to the lateral expansions of the central nodule in the Lyrata', 

 but there are no longitudinal lines in Lyrata; as in Diploneis, and in Diploneis there is no 

 punctate space between the median line and the prolongations of the central nodule. 



All the forms of this Section live in salt water, a few only (N. pyf/mcea and some varieties 

 of X forcipafa) living in brackish water; so that the forms of this section of Navicula are vei-y 

 characteristic of pureh' marine deposits. In the oldei' deposits only few occur. From the Barbados 

 deposit I have seen only one specimen, of a species akin to N. Barbito.s. In the Oamaru deposit 

 there are a few peculiar forms, one of which has the terminal fissures in contrary directions, 

 and another has almost straight terminal fissui'es. In the deposits of the mioc^ne or oligocene age 

 forms of Lyratte begin to become numerous, one of the most frequent being N. prcetexta. In the 

 present age, forms of this Section are very frequent, and the same species have a very wide range 

 of distribution. 



Most species of this section being transitional and their characteristics subject to great 

 vafiation. it is e"xtremely difficult to construct a satisfactory artificial key. Still I believe the 

 following will be of use. 



Artificial key. 



\ 



3 I 



^ I Lateral areas uniting with the central area 2. 



' \ — — not — — — — N. concilians Cl. 



Puncta forming straight longitudinal rows N. Durandi Kitton. 



undulating — — 3. 



Lateral areas linear 4. 



— broader in the middle 20. 



, ( Spaces between the areas and the median line striate 5. 



1 — -- — — — not — A'^. Reichardtii Grun. 



. j Central pores incrassate 6. 



^ \ - -not - 7. 



j Striae about 1.3 in 0,oi mm N- forcipata Grpn. 



6. •! — If) to 22 — — N. forcipata var. 



I — about 2f) — — N. pygmcea Kim,. 



Lateral areas short, not reaching to the margin 8. 



reaching — — 11. 



^ J Striae ending at the margin in double rows of small puncta N. riidis Cl. 



^- \ _ „„t __ _ _ _ _ _ _ 9. 



f ) Axial area narrowed at the ends and in the middle N. abrupta Greg. 



' ] — — indistinct 10. 



( Valve almost orbicular N. H. album Cl. 



lU. — elliptical N. connectens Grun. 



I (N. austrrilica A. S., N. Lyra var. atlantica, N. spectabilis, var., Rattrayi var. abbreviata). 

 . J Valve indented in the middle N. distenta A. S. 



1 -^ not - - - 12. 



. . j Valve narrow, linear with rounded ends N. Samoensis Grun. 



1 — broad, elliptical to' elliptic-lanceolate 13. 



.,,, I Puncta of the median strisB confluent towards the central nodule . . . . N. genifera A. S. 



{ 



not 14. 



J Punc 



[ Axial i)art of the valve elevated towards the ends N. Barbitos A. S. {N. Lyra. 



14. < '•. subcarinata). 



\ — — ~ — not 15. 



-p. /Lateral areas very distant from the median line N. diffiuens A. S. 



10. 



f Later a 



not — — — — 16. 



