BRITISH DIATOMACE>E. 55 



that any essential differences exist between this species and other 

 Orthosira. The " crenulations," noticed by Kiitzing and Thwaites, 

 are probably owing to a slight inflection at the suture (in younger 

 specimens) of the margins of the valves : even this inflection is, I fear, 

 only apparent, and dependent mainly upon the adjustment of the 

 light, as Mr. West gives little indications of it in his drawings, which 

 are executed with scrupulous fidelity. 



Adopting the genus Orthosira, I am therefore constrained to 

 reject that of Aulacosira, and divide the species formerly included 

 under the common name of Melosira into two genera ; the first, Me- 

 losira, known by the more or less rounded outline of the valves, and 

 the geminate union in its frustules ; and the other, Orthosira, by 

 having the junction- surfaces of adjacent valves plane, or truncated, 

 the line of junction usually marked by circlets of denticulations or 

 spines, and the frustules continuously united. 



It is probable that differences exist in the formation of the sporangia 

 of the two genera ; as these bodies, very frequent in Melosira, are of 

 rare occurrence in the other genus, and present peculiarities in the 

 single native species in which they occur, which would seem to indi- 

 cate specialties in the function concerned in their production. These 

 peculiarities will be noticed elsewhere, and are not, I fear, in the pre- 

 sent state of our knowledge, available as the groundwork of a correct 

 classification. 



]. Melosira nummuloides, Kiitz. Ordinary frustules globular; 

 sporangial frustules spherical, with the polar diameter shorter than 

 the equatorial, all geminate ; valves hemispherical, furnished with 

 a subcentral keel or flange of silex. Breadth of ordinary fila- 

 ment -0005" to -0008". Breadth of sporangial filament -0011" to 

 •0013". v.v. 



Var. 6. Mucus-cushion much developed. 



Var. y. Keel often obscure, or absent. 



Kutz. Bacill. iii. 3. ad specim. quae dedit cl. De Brebisson. Grev. B. F. p. 401 . 

 Ralfs, Ann. vol. 12. pi. ix. 1. Gallionella nummuloides, Ehr. Inf. x. 3. 

 & xi. 1. Prit. Anim. xiv. 14. Fragilaria nummuloides, Lyng. Tent, 

 lxiii. c. Conferva nummuloides, Dillw. Conf. p. 43. tab. B. Melosira 

 moniliformis, Ag. Syst. p. 8. ad spec, authen. in herb. Grev. Melosira 

 salina, Kiitz. Bacill. iii. 4. sic cl. Auctor in lit. Jul. 1851. Var. 8. Me- 

 losira salina, 8. concatenata, Kiitz. Bacill. iii. 5. & xxi. 5 b. j-^Cy ^~~K~",-^ 



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