60 BRITISH DIATOMACE^E. 



spherical ; valve with large irregular cellules, and a deep submedian 

 sulcus or depression ; junction-surfaces striated ; strias radiating. 

 Breadth of filament -0006" to -0018". v.v. 



Melosira sulcata, Kutz. Bacill. ii. 71 Gallionella sulcata, Ehr. Microg. 

 passim ? 



Marine : very generally distributed in deep water. Coast of Sussex ; Cork 

 and Kinsale Harbours, &c, W. Sm. Coast of Norfolk, Mr. Brightwell. 

 Frith of Clyde, Mr. Hennedy. Pembroke Harbour and River Thames, Mr. 

 Capron. River Tyne, Mr. D. Oliver. (Cherbourg, M. DeBrebisson. Medi- 

 terranean, W. Sm. Mauritius, Dr. Arnott. Ceylon, Dr. Kelaart.) 



Plate LIII. 338. 



3. Orthosira Dickieii, Thw. Filaments direct ; cell-cavity globular 

 or subspherical ; valves with a subsutural circlet of puncta ; junc- 

 tion-surfaces obscurely punctate. Breadth of filament *0005" to 

 •000S". v.v. 



Thw. Ann. 2nd ser. vol. 1. pi. xii. E. Prit. Anim. xxiv. 29. 



Fresh water. Cave near Aberdeen, Dec. 1847 and Nov. 1851, Dr. Dickie. 

 In the same locality, Aug. 1853, Dr. Redferti. 



The very singular process which takes place in this species, and which 

 Mr. West has accurately figured in its various stages of development in 

 Plate LIE, is regarded by Mr. Thwaites as the formation of the sporangial 

 frustule, and has been lucidly described by him in the volume above quoted. 



It is possible that this is the correct signification of these appearances, but 

 unhappily in no other species of this genus has there been found any analogous 

 process ; and this mode of development, in the formation of sporangia, stands 

 alone and unsupported : this is a serious difficulty in the way of admitting 

 Mr. Thwaites's conclusions. Another arises from the mode in which self- 

 division takes place in the " sporangium," subsequent to its formation. It 

 will be seen that, after the formation of a number of concentric rings of silex, 

 the sporangia assume an elongated fusiform shape, and upon the cessation 

 of this ring-development, an ordinary frustule makes its appearance, occupy- 

 ing the central portion of this fusiform body, but leaving the attenuated ex- 

 tremities unemployed. 



This appears to be a course quite at variance with the usual mode of spo- 

 rangial growth throughout the class, and throws doubt over the whole inter- 

 pretation given to these phaenomena. 



I am therefore disposed to refer the process to that mode of abnormal 

 development I have noted as occurring in Meridion circulare, M. constric- 

 tum, Himantidium Soleirolii, Odontidium anomalum, and Achnanthes sub- 

 sessilis. I would particularly refer to the last, in Plate XXXVIII. 302*, 

 as presenting phaenomena closely analogous to the present, and to the expla- 



