BRITISH DIATOMACEZ. 
Rennes 
Section I. FRUSTULES NAKED. 
Family I. NAVICULEZ. 
» 
Frustules free, solitary, more or less elongated, straight 
or curved; extremities symmetrical. Valve more or less 
convex, straight or sigmoid, and divided into two equal 
halves by a longitudinal median line having a central and 
terminal nodules ; surfaces of the valve on both sides of 
the median line covered partially or completely with hemi- 
spherical nodules arranged in transverse, oblique, or longi- 
tudinal lines, so as to form série, which are either granular . . 
or costate, according to the aggregation or segregation of 
the nodules. 
The original researches of Mr. Wenham®*, recently con- 
firmed by the more complete observations of the Rev. J. B. 
Readet, have demonstrated that the markings on the valves 
of the Naviculee are produced by hemispherical nodules 
projecting from the surfaces of the valve and arranged in 
lines inclined at varying angles to its longitudinal axis 
and to each other; the siliceous hemispheres on the 
a) 
* Quart. Micr. Journ, vol. iii. p. 244, and Trans. Micr. Soe. n. s. 
vol. viii. p. 145. 
+ Monthly Micr. Journ. July 1869, 
