BRITISH DIATOMACE^E. 19 



to leave this interesting addition to our native forms for the present united 

 with Odontidium, to which it seems allied through O. Tabellaria and 

 O. mutabile. 



Snpp. Plate LX. 373. Var. /3. Plate LX. 3/4. 



7- Odontidium? parasiticum, n. sp. Frustules rarely cohering, 

 attached by their extremities ; valves direct, elliptical, and acute. 

 Length of frustule -0005" to -0009". v.v. 



/3. Valve with a central constriction, v.v. 



Fresh water : parasitic on Nitzschia sigmoidea. Queen's Park, Edinburgh, 

 April and July 1854, Dr. Greville. 



A minute form, which I place with doubt in the present genus, having 

 never seen any approach to a filamentous union among its frustules. It is 

 scarcely siliceous, and may prove eventually to be the early condition of 

 some other species. 



Supp. Plate LX. 375. 



Genus 34. DENTICULA, Kutz. 



Frustules quadrangular, or elliptical, direct, rarely united into fila- 

 ments, free; valves elliptical, direct, striated, with pervious or 

 interrupted costae. 



I have already said that the present genus is with difficulty 

 distinguished from the last by any other character than that of the 

 more imperfect tenacity of its filament ; with no other is it likely to 

 be confounded except Diatoma, and that only in a fossil or prepared 

 state : when living, the coherence of the frustules at their angles is a 

 character which at once distinguishes the latter genus. 



1 . Denticula obtusa, Kutz. Frustules linear, truncate ; V. lanceo- 

 late, or linear, and attenuate towards the extremities, which are 

 obtuse; striae delicate, 36 in *00l''; costae unequally distant, about 

 12 in -001". Length of frustule -0006" to -0025". v.v. 



Kutz. Bacill. xvii. 14? Raben. Siissw. Diat. i. 8. 



Fresh water. Penzance, Mr. Ralfs. Aberdeenshire, Dr. Dickie. Rescobie, 

 Forfar, Mr. P. Grant. Ayrshire, May 1853, Dr.Landsborough. Braemai, 

 Dr. Balfour; communicated by Dr. Greville, Sept. 1854. 



Tbe reference to Professor Kiitzing is made with some degree of hesitation, 



c 2 



