8 THE STRUCTURE AND LIFE 



concave on the inside. When examined with good objectives of sufficient 

 resolving power and in media of high refractive index, all, or nearly all, 

 appear ornamented with designs or stria? running in different directions. 

 The best objectives, however, show that these stria? are illusory, and that 

 in reality they are alveoles in the thickness of the valves ( J ), the regular 

 arrangement of which gives the appearance of stria?. 



Many diatoms show an increase in the thickness of the valves, either 

 at the centre of figure alone, or, in addition to this, at the two extremities; 

 this increase of thickness is called a nodule (fig. 4). These nodules are 

 often connected together by a longitudinal line called a raphe or median 

 line. When the nodule in the centre is considerably enlarged so as to 

 spread laterally over all or part of the valve, the name of stauros is 

 substituted for that of nodule. 



The stria? do not always occupy the whole surface of the valve ; they 

 are very often absent near the median rib, and also round the central 

 nodule, where their absence may give rise to the semblance of a stauros 

 ( pseudo-stauros) . 



The unstriated part of the valve is called by English diatomists the 

 white or blank space, for which expressions Manoury has proposed to sub- 

 stitute that of mcsorhabde. The mesorhabde, which is faint or absent in nuny 

 diatoms, is highly developed in others, such as Navicula cardinalis, lata, &rc. 



In this work we have invariably used the terms zone or hyaline area 

 when referring to this plain portion of the valve. 



The surface of the valves is called the valve or valvular face or side 

 view of the frustule or simply the valve ; and the part of the frustule 

 corresponding with the connecting zone is called the frontal face, frotit, 

 girdle or zonal view or principal face. 



Some authors (Rabenhorst, &c.) have reversed the terms principal face 

 and lateral face, and we should greatiy approve of their idea if it were 

 not for the confusion which would necessarily result now that the previous 

 terms having been used by such eminent authors as Kutzing, Smith, 

 Gregory, Greville, &c, have been generally adopted. 



It is preferable to select the expressions valve face and girdle or zonal 

 face, which can leave no doubt in the reader's mind. These words which 

 we proposed in the third edition of our work on the Microscope have 

 been generally adopted, and we shall continue to employ them. 



( ' ) It was generally assumed, until recently, that the valves of diatoms were covered with hemi- 

 spherical beads in relief. We shall show hereafter that these so-called beads are cells hollowed 

 out of the thickness of the valve. We shall nevertheless continue to use the words beads, striae, 

 and costae, after fixing their true meaning, as they have passed into common use, and to abandon 

 them would introduce confusion in the descriptive part. 



