in common use as Test -Objects. 127 



Now, every authority with whom I am acquainted starts off 

 by admitting as an axiom that the lower the power under which 

 the structure of any object can be made evident, the less chance 

 is there of error. To dispute this would be absurd. The same 

 authorities admit, moreover, that under a moderate power (say 

 of from 300 to 500 diameters) the lineation of P. angulatum 

 appears in the form of obliquely or diagonally arranged lines, 

 placed at an angle of about 60 with the transverse section of 

 the valve, and enclosing "lozenge-shaped" spaces between their 

 intersections. And, strange to say, in some of our most elabo- 

 rate works, the greatest pains appear to have been taken to 

 show the beautifully defined character of these lines, under the 

 powers just indicated, even to the extent of employing specially 

 prepared polished paper for the engravings which represent it. 



Analogy has been pushed a little too far in this instance. 

 There are cogent reasons, open to every observer of the Diato- 

 macese, why we ought not to expect the same markings in the 

 Naviculoid as in the discoidal and allied forms. It would be 

 vain, for example, to endeavour to reduce such as are found on 

 Surirella or Pinnularia to the same category as those of Isthmia, 

 Biddulphia, or Triceratium. No forms can be more distinct in 

 this respect than these last named. Their typical structure, as 

 observed in T. favus, consists of depressed spaces, which vary 

 from true hexagons in their horizontal section to somewhat 

 irregular figures, such as we find in Eupodiscus Argus and Isth- 

 mia enervis, where, in addition to mere irregularity in outline, 

 the walls of the depressed spaces are studded with minute pro- 

 jections, giving them a ragged appearance under sufficiently high 

 magnifying power. In pressing either of these into our service, 

 therefore, simply because they are of dimensions large enough to 

 be most readily dealt with, a serious error has, in my humble 

 estimation, been committed. But it is a remarkable fact that 

 T. favus actually supplies us with markings strikingly similar 

 to those of P. angulatum, not on its valves, but on its connecting 

 zones. A very little care is requisite, in the case of the larger 

 varieties, to show that the markings of this portion of the frus- 

 tule are, as in the case of P. angulatum, diamond-shaped eleva- 

 tions, each of which presents four clearly defined facets, inclined 

 to the surface of the valve at an angle probably of about 25, and 

 yields, therefore, four distinct series of lines, two of which, the 

 diagonal, are most clearly marked, for reasons presently to be 

 given ; whereas the other two series (namely, the transverse and 

 longitudinal) are more nearly approximated to each other, and 

 much less palpable. 



In using direct light, either with or without an illuminating 

 combination, for the resolution of these markings, it is almost 



