20 BRITISH FRESHWATER RHIZOPODA. 
Diameter of test about 20 p, height about one half 
the diameter. 
Habitat.—Aquatic ‘vegetation. Rare. 
Eneianp.—Norfolk (Brightwell) ; Hampstead ponds, 
Middlesex (Pritchard). 
TreLANp.—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 Cyclotella, to which, had it been a diatom, 
it would have been most nearly allied. He first 
Fig. 165.—Pyzidicula operculata: u, 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 Brébisson in treating it as an 
autonomous genus. Hhrenberg then placed it doubt- 
fully under his new genus Galionella, suggesting the 
name Pyzxidicula, 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 Brébisson 
have described and figured the Cymbella or Frustulia 
operculata as the type of this genus, and correspond- 
ing perfectly with the foregomg generic character, 
Kuetzing himself, without acquainting us with the 
* Quoted from the translation of his “Sulla animalité delle Diatomee” 
published by the Ray Society in ‘ Botanical and Physiological Memoirs.’ 
