n v. T. TLEVE, SYNOPSIS OF XAVirrLOIT) DIATOMS. 



puncta. In many forms there is aii interiör stratum, which may be isolated from the alveolar 

 network. It is frequentlj^ found in preparations of Trachyncis Debi/i and ailied forms, and has 

 beeii fignred in A. Schmidts Atlas Pl. XL VIII f. 23. Also in Karicula (lisfaiis this interiör stra- 

 tum occnrs frequently in an isolated form. 



In the most c.omplicated cases, as in Trafliynvis, we ma}^ distinguisli three different 

 sti'ata, thus 



1. Tlie porons or ocelliferons interiör stratum. 



ä. The alveolar stratum of reticnlating costse. 



3. The exteriör, minutely punctate stratum. 



The structure of the valves presents characteristios wliieh are of great importance for dis- 

 tinction both of speeies and genera. The coarseness of the strufture may varv in tlie same .species, 

 though in most cases sueh variation is less thau is usually believed. The direction of the stri;t, 

 the arrangement of their puncta, are characteristios subject to only very sliglit variation in the 

 same speeies and in groups of ailied forms. 



Cell-conieids. As is well known, the chromatophores of the diatoms present a great varia- 

 tion in different tribes, consisting in sonie of numerous granules, in others of one or two plates. 

 To the latter type belong almost all the naviculoid diatoms, which have either one or two chromato- 

 phore-plates. The position and the form of these plates appear to be constant for groups of 

 ailied speeies, as is proved by the excellent researches of Pfitzer. But the cell-contents are 

 known only in a very limited number of forms, and the characteristios dependent on the cell- 

 contents are, for the greatest mimber of forms, yet to be studied. The same is also the case 

 with the reproduotion by means of auxospores, and the characteristios, derived from the living 

 cell, cannot for the present be used in the systematic arrangement of speeies, as thej'' are too 

 little known. 



The characteristios derived from the structure of the valve are of the greatest im- 

 portance in the definition of genera and families, and next to those, in my opinion, are tlie 

 presenoe or abseuce of longiti^dinal lines, and the nature of the non-striate parts of the valve, 

 or the areas. 



An ideal system should take in oonsideration the evolution of the different forms, but in 

 the present fragmentary state of our knowledge, suoh consideration cannot be more than an ap- 

 proximation wich may beoome closer to the truth as our knowledge of the forms becomes more 

 extensive and perfeot, The cjuestion which of the groups of forms are the highest and lowest may 

 be answered by considering as the lowest those in which the characteristios are the least developed, 

 and as the highest those in which the characteristios have attained their greatest evolution. But 

 a greater development in one direction usually involves the slighter development, or oblitera- 

 tion, of otlier characteristios, so that one group may be the highest in some respeots, and 

 another in others. The ohanges of the organisms do not alwa3\s indioate progress, but are 

 frequently retrograde, especially where the mode of life has beoome parasitical. A natural 

 system must consequently take account uot only of the upward evolution, but also of the 

 downward. 



As the known diatoms probably represent nierely a fraotion of those, which exist and 

 have existed, it will be necessary in constrncting a natural system to till the gaps with con- 

 jectures. 



There can be no doubt that the naviculoid diatoms are most nearly conuected with the ISitz- 

 chiccB among the diatoms without median line. The genera Tropidoncls and Amphiprora are very 

 closely ailied to several forms of Nitzc/na. We meet in that genus with the cariuated as^-mmet- 

 rical valves and the wing of Tropidoneis, also the complex zone and the carinal puncta of Amphi- 

 prora. In constrncting a natural system then we may arrange the genera and groups in order of 

 their relationship to the two genera above mentioned. and the following .soheme sliews how I .sup- 

 po,se the different groups of the naviculoid forms to be related. 



