o p. T. CLEVE, SYNOPSIS OF NAVICITLOin DIATOMS. 



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

 network. It is frequently found in preparations of Trachyneis Dehyi and allied forms, and has 

 been figured in A. Schmidts Atlas PI. XL VIII f. 23. Also in Navicula cUstans this interior stra- 

 tum occurs frequently in an isolated form. 



In the most complicated cases, as in Trachyneis, we may distinguish three different 

 strata, thus 



1. The porous or ocelliferous interior stratum. 



2. The alveolar stratum of reticulating costee. 



3. The exterior, minutely punctate stratum. 



The structure of the valves presents characteristics which are of great importance for dis- 

 tinction both of species and genera. The coarseness of the structure may vary in the same species, 

 though in most cases such variation is less than is usually believed. The direction of the strise, 

 the arrangement of their puncta, are characteristics subject to only very slight variation in the 

 same species and in groups of allied forms. 



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

 tion in different tribes, consisting in some 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 

 allied species, 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 characteristics dependent on the cell- 

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

 with the reproduction by means of auxospores, and the characteristics, derived from the living 

 cell, cannot for the present be used in the systematic arrangement of species, as they are too 

 little known. 



The characteristics 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 the 

 presence or absence of longitudinal lines, and the nature of the non-striate parts of the valve, 

 or the areas. 



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

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

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

 extensive and perfect, The question which of the groups of forms are the highest and lowest may 

 be ansM'ered by considering as the lowest those in which the characteristics are the least developed, 

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

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

 tion, of other characteristics, so that one group may be the highest in some respects, and 

 another in others. The changes of the organisms do not always indicate progress, but are 

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

 system must consequently take account not only of the upward evolution, but also of the 

 downward. 



As the known diatoms probably represent merely a fraction of those, which exist and 

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

 jectures. 



There can be no doubt that the naviculoid diatoms are most nearly connected with the Nits- 

 chiece among the diatoms without median line. The genera Tropidoneis and Amphiprora are very 

 closely allied to several forms of Nitschia. We meet in that genus with the carinated asymmet- 

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

 prora. In constructing 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 scheme shews how I sup- 

 pose the different groups of the naviculoid forms to be related. 



