THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



SUPPLEMENT TO VOL. XIL DECEMBER 1843. 



LIV. — On the British species o/Meridion and Gomphonema. 

 By John Ralfs, Esq., M.R.C.S., Penzance*. 



[With a Plate.] 



Meridion, Ag. {Eh.) 

 Erustules cuneate, united so as to form segments of circles or 



spirals. 



This genus, together witli Sty liar ia, Gomphonema and Licmo- 

 phorttf form a group (the Styllariece of Agardh) distinguished by 

 the triangular form of the frustules, which have their smaller 

 ends directed towards a common centre. The frustules in this 

 group have a central and two lateral portions as in Diatoma and 

 Fragilaria, in which genera cuneate frustules are also occasionally 

 met with. Indeed Diatoma cuneatum, Eh., is said to have all its 

 frustules cuneate ; in this case, therefore, the resemblance between 

 solitary frustules will be considerable. But in Fragilaria or Dia- 

 toma, when two or more cuneate frustules are united, the alter- 

 nate frustules have their smaller ends in opposite directions, and 

 hence their filaments are linear ; whilst they are attached, if at 

 all, only by their basal frustule. In this group, on the contrary, 

 as the smaller ends are in the same direction they point to a 

 common centre, and when the plant is stipitate each frustule is 

 attached to the stipes. 



The frustules have generally two puncta at the broader end, 

 and sometimes two others, but more obscure at the smaller end. 

 The lateral surfaces are attenuated at the base, which usually dif- 

 fers somewhat from the upper end; but in the group to which 

 Fragilaria belongs they are similar at both ends, even when the 

 frustules are cuneate. 



In a young state species of Meridion resemble Styllaria, but 

 as the frustules increase in number and do not separate, they 

 form more or less complete segments of circles ; whilst in Stylla- 

 ria rarely more than two, and never more than three, frustules are 

 united together. 



1 . M. circulare, Ag. Frustules united into plane segments of circles ; 

 lateral surfaces clavate with distant, strongly marked striae, the 



• Read before the Botanical Society of Edinburgh. 



Ann. &f Mag. N. Hist. Vol. xii. Suppl. 3 I 



