1884.] 



MICROSCOPICAL JOURNAL. 



105 



clearness and boldness of its marking 

 make it a very profitable subject for 

 careful examination. It is easy to 

 get somewhat varied appearances by 

 dirterent uses of the light and changes 

 of focus of the objective, but if we 

 use the narrow central pencil of light 

 and care in focussing, its characteris- 

 tics will be found uniform and unmis- 

 takable. Its lyrate hyaline figure in 

 the middle of the valve takes the pink 

 tint. The dots are found to be be- 

 tween costiE which are fully as wide 

 as the dotted interspace, and these 

 have the same color as the lyrate fig- 

 ure. Find a broken shell and focus 

 carefullv upon the broken margin. 

 Oftentimes the costae w^ill be found to 

 project beyond the interspace, show- 

 ing its greater strength, and confirm- 

 ing the evidence to this effect which 

 is found in its deeper color. When 

 the focussing gives us the costie as 

 well-defined ribs of even width, and 

 a broken edge is also most sharply 

 defined, the dotted interspace will ap- 

 proximate to a ladder-like appear- 

 ance, the dots having a sub-rectangu- 

 lar form, and being separated from 

 each other by septae considerably 

 narrower than the costae between 

 which they lie. The term ^ sub-rec- 

 tangular' which I have used must not 

 be taken too literally, for the figure 

 of the dots is that of a circle some- 

 what flattened on four sides. Assum- 

 ing that the median line is a groove 

 in the valve, and focussing upon it 

 so that the light coming through it 

 shall correspond nearly to the general 

 field, it will then be found that the 

 dots nearest this line and most per- 

 fectly in the same plane show the 

 same color — an item of evidence that 

 they, too, are thin places in the shell. 

 But the. line of fracture gives still 

 stronger proof. I have before me a 

 broken valve of A", /yra, in which a 

 segment is entirely gone, bounded by 

 the median line for, say, half the dis- 

 tance from the end of the shell to the 

 central nodule. Then the broken 

 margin runs irregularly off' to the rim 

 of the shell. On the other side a wide 



crack extends diagonally from the 

 median line a short distance, then 

 rims straight out to the rim. This 

 crack (examined with a ^^ objective) 

 zigzags through the dots in the first 

 part of its course, and in the straight 

 part runs indisputably through the 

 dots and between the straight costa;. 

 The broken edge of the other side of 

 the shell shows with equal clearness 

 that the fracture is through the dots. 

 I have many such cases noted, with 

 great varieties of fractures, but all 

 indicating the same fact in regard to 

 the structure, viz., that the dots are 

 the thin and weak places in the valve. 



Another point to be noted is that 

 whilst the radiant costie of A^. /yra are 

 straight, making also straight trans- 

 verse striation when viewed with a low 

 power, the longitudinal septie between 

 the dots are net regularly continuous ; 

 consequently when light is thrown 

 transversely across the shell a low 

 power shows longitudinal striae, but 

 wavy instead of straight. This is also 

 the case with the striation ofJV. fir ma ^ 

 N. ctispidata^ N. rhoinboides^ and 

 Frustulia Saxonica when examined 

 with high powers, and with the 

 Nitzschias of the form of iV. seal arts,, 

 N. linearis^ etc., of which the coarser 

 specimens show distinct lines of punc- 

 tae between parallel costae. It is 

 characteristic, too, of the difference 

 between the transverse and longitudi- 

 nal striai of Surirella gemma. It is 

 certainly natural to conclude that the 

 similar phenomena are due to similar 

 structure. 



In naviculae having strongly radiant 

 costae, some, like N. pereg-rina^ show 

 a similar dotted structure between the 

 ribs, and in these cases the lines of 

 separation between the dots are also 

 much finer and less prominent than 

 the costae. In another class of navic- 

 ulae, of which IV. sculpta., Ehr., is an 

 example, the dots, whilst arranged in 

 lines, do not have thickened costse 

 between the rows, but are like sepa- 

 rate, sometimes elongated, punctae in 

 a shell of even thickness. In these, 

 however, as in A^. h'ra, the line of 



