BRITISH DIATOMACE^E. 15 



Genus 33. ODONTIDIUM, Kiitz. 



Frustules quadrangular, direct, united into a filament, at first attached, 

 at length free ; valves elliptical or cruciform, costate, costae pervious 

 or interrupted. 



The frustules of Odontidium are distinguished from those of the last 

 genus by their straight outline and conspicuous costae, from those of 

 Fragilaria by the latter character, and from those of Denticula by 

 their more tenacious adherence and striated valves. It must however 

 be acknowledged that there is little to separate the last-named genus 

 from our present one, and I should be disposed to unite the two, 

 were there not in the general habit of the living frustule charac- 

 teristics which enable the observer to assign them to their respective 

 genera; Odontidium often occurring in filaments of considerable 

 length, and in some cases of no little tenacity ; Denticula invariably 

 in fragmentary portions rarely presenting more than two or four in 

 union, and that union apparently of the feeblest kind. The surface 

 of the valve in Denticula is also very distinctly striated, while in 

 Odontidium the striae, if present, are obscure, and elude detection by 

 all but the most powerful and carefully-adjusted glasses. The 

 appearance of minute teeth on the F. V. of the frustule, which arises 

 from the terminations of the costae, and from which Professor Kiitzing 

 has derived the generic appellation of Denticula, is however far more 

 conspicuous in some of the species belonging to Odontidium ; and 

 the two genera are so alike in their leading features, and their 

 appellations so similar in meaning, that it may be left to future 

 observers to consider whether they may not without much incon- 

 venience be permanently united. 



1. Odontidium hyemale, Kiitz. V. elliptical, frequently attenuated 

 towards the obtuse extremities; costae pervious, from 5 to 12. 

 Length of frustule -0008" to -0014". 



Kiitz. Bacill. xvii. 4. Raben. Siissw. Diat. ii. 4. Fragilaria hyemalis, Lyng. 

 Tent, lxiii. E? Ralfs, Ann. vol. 12. pi. ii. 5. ex parte, et Hass. Alg. 

 xcv. 5. ex parte, ad specim. authen. in herb. Jenn. Fragilaria Con- 

 fervoides, Grev. B. F. p. 403. ad specim. authen. in herb. Grev. Harv. 

 Man. p. 197. Odontidium turgidulum, Kiitz. Bacill. xvii. 2. Raben. 

 Siissw. Diat. ii. 6. Fragilaria turgidula, Ehr. Inf. xv. 13. 



