BRITISH DIATOMACE^E. 7* 



Diameter of frond from -2" to -6". Length of frustule -0008" to 

 •0015". Breadth of V. -00035". v.v. 



Thw. Ann. ser. 2. vol. 1 . pi. xii. G. 



Fresh water. North River near Wareham, Dorset, July 1847 and May 1853, 

 W.Sm. 



This species bears a slight resemblance in the character of its frond to 

 Berkeley a fragilis, but differs in the form of its frustule and its freshwater 

 habitat. The globose frond originates in the same way as in Berkeleya, by 

 the coherence of the mucus-tubes or filaments ; but, unlike that species, the 

 mucus continues in the present to increase in volume, and always extends 

 beyond the extremities of the rows of frustules ; while in Berkeleya the fila- 

 ments are free at their terminations, being continued beyond the mass of mucus. 

 Conjugation has been observed in the present species, and takes place within 

 the frond precisely in the same manner as in the free forms described under 

 the first tribe. 



Plate LVI. 353. 



Genus 56. SCHIZONEMA, Agardh. 



Frond gelatinous or submembranous, filiform, or by coherence leaf- 

 like, usually much divided ; frustules naviculoid, arranged in one 

 or more files in linear series within the substance of the frond ; 

 valves elliptical, or lanceolate, striated. (All the species marine.) 



The fronds in this extensive genus were among the earliest Diato- 

 maceous organisms recognized by naturalists, and have been the per- 

 plexity of all subsequent observers. Their external form, size and 

 colour vary with age, season and locality, and in consequence any cha- 

 racters based alone upon these particulars are uncertain and deceptive. 

 Now these were the characters most relied upon by the earlier writers 

 in their specific arrangements, and hence the descriptions of one 

 author were often irreconcileable with those of another, and the 

 species became inextricably confounded. Later observers endea- 

 voured to clear up this confusion by a closer examination of the 

 structure of the frond, and it was thought by some that the species 

 might be arranged under two genera, as they presented one or more 

 series of frustules included within a single gelatinous or membranous 

 tube, or contained in secondary tubes, enclosed within a general 



