BRITISH DIATOMACEjE. 47 



1 . Amphitetras antediluviana, Ehr. Frustules cubical ; margins 

 on S. V. nearly straight, forming a square. Length of a side of 

 valve -0011" to -0042". v.v. 



j3. Margin of valve deeply concave ; frustule in consequence cruciform. 

 Distance between the adjacent angles of valve "0025" to •0062". v.v. 



Ralfs, Ann. vol. 12. pi. viii. 5. Kiitz. Bacill. xix. 3. Prit. Anim. xiv. 21, 22. 

 Var. (3. Kiitz. Bacill. xxix. 86. 



Marine. Torbay, June 1846 ; Poole Bay, July 1849 and Sept. 1854, W. Sm. 

 Haverfordwest, July 1854, Mr. Okeden. Ilfracombe, Mr. Ralfs. Torbay, 

 Mrs. Griffiths. Var. j3. Stomach of Crab, Professor Williamson. Ipswich, 

 Aug. 1852, Mr. Hodgson. Cumbrae, Feb. 1854, Mr. R. Hennedy. (Var. fi. 

 Salt Pans near Agde, South of France, May 1854, W. Sm.) 



It is probable that the above Var. /3. has a claim to be esteemed a distinct 

 species. I have not found well-marked specimens of the two forms in any 

 instance intermixed, and the deeply excavated sides, and, in consequence, 

 cruciform outline of the frustule, are as notable in the smallest as in the largest 

 individuals of the Mediterranean gathering. 1 have distributed specimens 

 of the latter under the name of A. excavata ; but, in deference to the opinion 

 of my esteemed correspondent Dr. Walker-Arnott, have determined to retain 

 it for the present as a variety of the more usual form. 



Plate XLIV. 318. Var. /3. Plate XLIV. 318 a'". 



Genus 46. BIDDULPHIA, Gray. 



Frustules compressed, adhering more or less perfectly into a con- 

 tinuous or zigzag filament; valves convex, elliptical, usually 

 spinose and cellulate ; cellules circular ; angles of valves produced 

 into rounded or horn-like processes. 



The frustules of Biddulphia are so various in outline, that it is 

 difficult to fix upon any constant generic characters descriptive of 

 their form ; they all, however, agree in having compressed frustules, 

 with the angles of the valves produced, and in presenting a cellulate 

 structure, not only in the valves, but also in the connecting mem- 

 brane. The latter is frequently projected beyond the suture, and 

 appears as an encircling band of silex standing out from the general 

 surface of the valve. Frustules of Eupodiscus, adhering after self- 

 division, often closely resemble Biddulphia ; they differ, however, in 

 the orbicular outline of their valves, and in the processes of the 



