Professor Smith's Conspectus of the Diatomacese. 115 



silicious frustules or skeletons, without any reference to the mode of 

 growth and habitat of the living organisms. As an example of 

 his method of treating the subject, it is enough to state that Cocco- 

 nema and Cymbella, the one a stipitate and the other a free form, 

 are grouped together, Cocconema being, in fact, abolished as a genus 

 and retained only as a synonym. And so Arachnoidiscus is placed 

 in the same tribe as Melosira, the first being invariably a free 

 form and the latter a mere integral portion of a long filament ; 

 and then, again, on the same principle, the frondose genera Schizo- 

 nema, Collotonema, and Encyonema are done away with, their 

 particular habits of gelatinous growths being ignored and the forms 

 of their frustules only taken into account. It is true that Professor 

 Smith follows Mr. Ralfs, and most modern diatomists, in this way 

 of looking at the matter, but it is still questionable whether the 

 plan of almost the oldest English diatomist, the Rev. W. Smith, is 

 not the best, as it certainly is the most natural, viz. that of 

 arranging the Diatomacese according to their habitat and method of 

 growth, whether free, stipitate, coneatinate, frondose, or gelatinous. 

 Our author, however, in vindication of his system, says that he has 

 not considered the conditions of growth sufficient to warrant the 

 formation of new genera, a long study of living forms having con- 

 vinced him that these characters are fleeting and not to be relied 

 on ; and he gives certain examples in proof of this statement. 



Professor Smith divides the order into three tribes ; the first 

 mostly bacillar, with distinct raphe, cleft, or median line, with 

 nodules ; the second, generally bacillar, with pseudo or false raphe, 

 or blank space ; and the third generally circular, with a crypto or 

 concealed raphe. His first tribe contains five families, the second 

 three, and the third seven; and these fifteen families are again 

 subdivided into one hundred and ten genera, exactly half of which 

 belong to the third tribe. To one or other of these genera the Pro- 

 fessor considers any new form may be ascribed. He has taken 

 great pains in collecting together every known synonym. On 

 looking over his index to the synonym register in the third 

 number of the ' Lens,' it will be seen at a glance what a vast 

 number of genera (189) are abolished, and apparently very pro- 

 perly in most cases. There are two or three familiar genera, how- 

 ever, whose extinction the old diatomists will scarcely relish, 

 though the author may be able to justify it — Campylodiscus, for 

 instance, is now relegated to Surirella, and, possibly, rightly so; 

 there are arguments, however, against the incorporation, as the 

 median line of the opposed valves of the former are at right 

 angles to each other, whilst they are parallel in the latter ; other- 

 wise the tout ensemhle on which Professor Smith chiefly relies is 

 pretty similar. As a general rule certainly Surirella is more or 

 less oval and flat, whilst Campylodiscus is circular and twisted : 



