20 BRITISH FRESHWATER RHiZOPODA. 



Diameter of test about 20 /*,, height about one half 

 the diameter. 



Habitat. Aquatic vegetation. Rare. 



ENGLAND. Norfolk (BrigJitwell) ; Hampstead ponds, 

 Middlesex (Pntchard). 



IRELAND. Clare Island, Caher Island, and mainland, 

 Mayo. 



This species was for many years considered to be a 

 diatom^ It will be seen from the synonymy that it 

 was first described by Agardh as a Frustulia, and then 

 placed by him in the genus Cymbella, to which it has 

 but little nearer resemblance. By Kuetzing it was 

 referred to Cydotella, to which, had it been a diatom, 

 it would have been most nearly allied. He first 



b c 



FIG. 165. Pyxidicula operculata : a, vertical view ; b, upper detached 

 part ; c, lower detached part, x 750. (After Brightwell.) 



considered that to be a sub-genus of Frustulia, but 

 afterwards followed Brebisson in treating it as an 

 autonomous genus. Ehrenberg then placed it doubt- 

 fully under his new genus Galiondla, suggesting the 

 name Pyxidicula, and soon afterwards described and 

 figured it in his great work ' Die Infusionsthiere ' as 

 Pyxidicula operculata, under which name it stands as 

 the type-species of its genus. 



Meneghini, in his synopsis of Kuetzing's ' Bacil- 

 larien oder Diatomeen,' after giving a diagnosis of 

 Pyxidicula, remarks* : " It is really surprising that 

 although Agardh. Kuetzing, Ehrenberg, and Brebisson 

 have described and figured the Cymbella or Frustulia 

 operculata as the type of this genus, and correspond- 

 ing perfectly with the foregoing generic character, 

 Kuetzing himself, without acquainting us with the 



* Quoted from the translation of his " Sulla animalite delle Diatomee " 

 published by the Ray Society in ' Botanical and Physiological Memoirs/ 



