DIATOMACEJE. 



313 



covered the beautiful longitudinal and transverse strice 

 (groovings) on the Pleurosigma hippocampus, or sea-horse 

 ship, No. 1. A curved graceful line runs down the shell, 

 in the centre of which is an expanded oval opening. 

 Near to the central opening the dots elongate crossways, 

 presenting the appearance of small short bands. The 

 Pleurosigma angulatum, No. 2, cornered little ship, was first 

 discovered in the Humber ; the lines upon its surface re- 

 semble the most elegant tracery, which are resolvable into 

 raised minute dots. The markings are seen to be longi- 

 tudinal, transverse, and oblique. Some observers believe 

 the spaces to be hexagonal in shape ; but this is certainly 

 a deceptive appearance, arising from bad illumination or 



Fig. 182. 



1, Pleurosigma angulatum. 2, Portion of the same, magnified 1200 diameters. 

 3, Portion of P. formosum, magnified 5500 diameters. 



aberration in the glasses. Mr. Wenham's photograph of 

 this shell is an exaggerated imperfection, produced by an 

 error of foci in his camera lenses. 



The Green Navicula, first found by Dr. Mantell in a 

 pool on Clapham Common, is about the 100th part of an 

 inch in length. In this specimen the ribbed division is 

 distinctly seen, extending the whole length of its shell. 



Eunotia. In this species a furrow is seen the entire 

 length of the shell, from which spring numerous ribs ex- 

 tending towards the edges ; of these, eight may be counted 



