Naviculacea.'] infusoeial animalcules. 371 



of view, as the filaments are turned round, they all appear like 

 puucta. 



" * Filaments moniliform, frustules united in pairs. Species G. . 

 nummidoides, G. Borreri, and G. glohifera. 



" * * Filaments not moniliform, irustules cylindrical. Species G. 

 arenaria, G. varians, and G. aurichalcea. 



"* * * Filaments very slender, joints obscure. Species G. ochracea.'" 



Mr. Thwaites has proposed (.inn. Nat. Hist., 1848) another 

 arrangement of Melosira — subdividing it indeed into three genera — 

 viz., Aulacoseira, Orthoseira, and Melosira. This subdivision he would 

 base on differences observable both in the character and position of 

 the sporangia, and in the form and structure of the frustules them- 

 selves. 



The genus Melosira (says Mr. Thwaites) as it stands, after this 

 removal of some of its species, will include all those whose frustules 

 are in any degree convex at their extremities, and have the central 

 line indicating the place of future fissiparous division. It will 

 probably be found expedient to separate Melosira arenaria (Moore, 

 see page 376) from its present congeners, when its sporangia have 

 been discovered. 



Besides multiplying by fission, the Melosira GaJlioneUa (Ehr.), 

 develope new frustules by the formation of sporangia ; and, although 

 these sporangia are not the result of an evident conjugation, or mix- 

 ture of eudochromes of two frustules, as witnessed in many of the 

 Diatomeoi, yet Mr. Thwaites concludes, that the difference in the 

 phenomena, though structural, is not physiological. In tliis genus, 

 a change takes j^lace in the endochi'ome of a single frustule — that is, 

 a disturbance of its previous arrangement, a moving towards the 

 centre of the frustules, and a rapid increase in its quantity ; subse- 

 quently to this it becomes a sporangium, and out of this are developed 

 sporangial fr'ustules as in other JJiatoniece. 



In the system of Kiitzing, this genus gives name to the family 

 Melosirce, which comprise the genera Cyclotella, Pyxidicula, Fododiscus, 

 Podosira, and Melosira. 



The same observer states that Melosira is attached by a soft, gela- 

 tinous pedicle, proceeding fi-om the middle apertui-e of the veiitral 

 surface, a characteristic of this genus. 



