10 Prof. W. Smith on the Diatomacese of the Pyrenees. 



when cautioning the inexperienced observer against relying upon 

 size or outline merely as the basis of specific distinctions. Had 

 filaments with the above deviation been the only ones occurring 

 in the Cambuscou station, an abnormal state of the frustule 

 might have been regarded as its permanent form, and a specific 

 character been founded on what was merely a morbid condition 

 of its growth. 



Plate I. fig. 10. Odontidium hyemale. Var. ^. a, S. V. of frustule ; 

 a', S. V. of deformed frustule ; h, front view of filament. Fig. 1 1 . a & 

 o', side views of frustule ; b, F. V. of filament. 



Odontidium informe, n. sp. S. V. of frustule elliptical, with an 

 irregular inflation at the centre, and hence subcruciform. 

 Costse not reaching the centre of the valve, 18 in '001". 

 Length of frustule -0007". Greatest breadth -0003". v.v. 



Fresh water, Lac Gave de Lizez, and Gave de la Reine near 

 Cauterets. 



The nearest ally of this species is O. Harrisonii, Synop. B. 

 Diatomacese, vol. ii. p. 18 ; but the normal form of the latter not 

 occurring in the Pyrenees, it is probable that the present is 

 entitled to rank as distinct. Odontidium Harrisonii , var. /9, 

 Synop. vol. ii. pi. 60. fig. 374, is also a frequent form in my 

 Pyrenean gatherings, and is also in all probability a distinct 

 species, though, from its great similarity to the Hull frustules, I 

 have, with most other observers, hitherto regarded it as a mere 

 variety. 



Plate II. fig. 12. Odontidium informe: a, b, c, side views of frustule; 

 d & d', front views of ditto. 



Diatoma Ehrenhergii, Kiitz. S. V. linear-elliptical, constricted 

 towards the slightly capitate extremities, costate. Costse 

 30 in -001". Length of valve -0011" to -0017". Greatest 

 breadth of ditto -00016" to -00023'^ v.v. 



Kiitz. Bacill. tab. xvii. 17; Sp. Alg. p. 17. 



The inflation in the centre of the valve, with the constriction 

 near the subcapitate extremities, exactly correspond with the 

 figure and description given by Professor Kiitzing, and separate 

 the present species from D. grande of the ' Synopsis,' which is 

 moreover a larger form with coarser striae. 



Plate I. fig. 13. Diatoma Ehrenhergii : a &c b, side views of frustule ; 

 c, F. V. of ditto. 



Himantidiumt marinmn, n. sp. Filament tenacious. S. V. 

 slightly and regularly arcuate, extremities acute, costate. 



