24 



CONSPECTUS OF DIATOMACE^E. 

 THE GENUS AMPHORA. 



Prof. H. L. Smith. 



We have just received a further instalment of Professor Smith's 

 New Conspectus,* in which he treats of the genus Amphora. In 

 none of the genera of Diatomacece is the structure of the frustule 

 more difficult to understand ; owing to the unequal development of 

 the cingulum or connecting zone, the two valves of the frustule 

 are always visible at the same time. Professor Smith's explana- 

 tion will enable the student to comprehend the peculiar formation 

 of the frustules of the species of this genus. 



" Bearing in mind that all diatomaceae are built after the same 

 type, or are silicious boxes, as I. have already indicated in the 

 preface to the ' Synopsis,' a reference to the following diagrams 

 will make the structure of Amphora plain. If we commence with 

 a typical navicula form, as in figure 1, presented in side view, we 

 have the median line (raphe) dividing the valve symmetrically. 



Passing to figure 2, we have the typical Cymbella, the median line 

 being nearer to one margin than the other, or dividing the valve 

 un symmetrically. The more convex margin is termed the dorsum, 

 and the other the venter. Although these are objectionable terms, 

 yet, as they have been extensively adopted, I shall continue to use 

 them. If we pass now to figure 3, we have a more decided depar- 

 ture from the navicula, in the curved raphe, and the more or less 

 curved ventral margin. Let us look at these frustules in front 

 view ; a and b are the striated valves, with central nodule, while 

 the dotted lines c d represent the lines of suture (in all the 

 figures the lines are dotted); fig. 5, the end view of the same frus- 

 tule. While the valves, as seen in figures 4 and 5, are slightly 

 convex, the sutural zone, or hyaline part, which has upon it the 

 sutural lines, is of the same width at the two ends c and d, fig. 4, 



* Published in " The Lens," No. 2, vol. 2. 



