BRITISH DIATOMACE^. 75 



Genus 24. COCCONEMA, Ehr. 



Frustules stipitate, cymbiform ; valve with a submedian line, having 

 nodules at centre and extremities. 



The frustules of this genus differ in no respect from those of Cym- 

 bella (Genus 3), and are placed apart solely from the presence of a 

 stipes. Whether the development of this accessory be esteemed a 

 generic difference or not, it cannot be denied that it gives a notable 

 physiognomy to the plant, and affords a ready means of discrimi- 

 nation. The stipes seems to be an exudation from the frustule, 

 secreted only during the progress of self-division. It assumes, in the 

 present and other genera, a membranous consistency and filamentous 

 form ; while the same secretion is present in Cymbella and many of 

 the free Diatomaceous forms as a mucous stratum, and in others as a 

 gelatinous cushion or prolonged pedicel. 



Where it possesses a filamentous character it invariably branches 

 in a dichotomous manner, — a necessary consequence of the self-division 

 of the frustules, and the circumstance that each filament or branch is 

 the secretion of a separate frustule. Thus, when self-division is com- 

 pleted, the extension of the filament below the frustules is suspended, 

 a joint or articulation is formed at the base of the dividing frustule, 

 and each of the half-new frustules begins anew, in its progress towards 

 special self-division, the secretion of a new joint or internode ; and a 

 dichotomy is the result. Conjugation and the formation of Sporangia 

 in the present genus have been observed by Mr. Thwaites in C. lan- 

 ceolatum and C Cistula, and by myself in both these species and 

 C. parvum. 



1. Cocconema lanceolatum, Ehr. F.V. lanceolate, obtuse; V. elon- 

 gated, slightly inflated at the centre of the concave margin ; striae 

 moniliform, 21 in -001". Length -0043" to '0058". v. v. 



/3. Much elongated (Sporangial?). Length -OOSS"- to -0098". 



Ehr. Inf. xix. 6. Kiitz. Bacill. vi. 3. Dujar. Inf. xx. 4. Prit. Anim. iv. 195. 

 Hass. Alg. ci. 1. In Coivjvigation, Thw. Ann. vol. xx. pi. xxii. C. 



Fresh water: frequent. In Conj. River Froome, Dorsetshire, Mar. 1847. 

 Cockshut stream near Lewes, Ap. 1850. Plumpton, Sussex, Ap. 1852, W. Sm. 



