with descriptions of new genera and species. 167 



tion j and it is interesting to find that difi*erences are observable 

 in the character and position of the sporangia sufiicient to justify 

 a removal of some of the species from the genus Meloseira, in 

 which they are at present included by Professor Kiitzing in his 

 valuable work on the Diatomacece. A careful examination shows 

 too^ that, independently of the difference in the sporangia, there 

 is sufficient distinction between the frustules themselves to cha- 

 racterize the proposed new genera. 



Although it is very probable that hereafter it will be found 

 desirable to break up still further the genus Meloseira, it is pro- 

 posed at present only to separate from it — 1st. Those species 

 characterized by the absence in the frustule of an evident cen- 

 tral line indicating the place of subsequent fissiparous division, 

 but each frustule having two somewhat distant sulci or fossulse 

 passing round it — Aulacoseira. 2ndly. Those species, the frus- 

 tules of which are not at all convex at the extremities, and which 

 therefore form by their close contact an uninterrupted cylindrical 

 filament ; each frustule is marked with a central line and its in- 

 ternal cavity is spherical or subspherical — Orthoseira. 



These two new genera may be defined as follows : — 



Aulacoseira. — Cellulis cylindricis bisulcatis extremitatibus plus 

 minusve rotundatis in fiiamenta concatenatis. 



Typ. spec. Meloseira crenulata, Kiitz. = M. orichalcea, Ralfs. 



Orthoseira. — Cellulis exacte cylindricis linea centrali notatis 



in fiiamenta cylindrica connexis ; cavitatibus internis sphsericis 



vel subsphaericis. 



Typ. spec. Meloseira americana, Kiitz. 



The genus Meloseira, 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 in- 

 dicating the place of future fissiparous division. It will probably 

 be found expedient to separate Meloseira arenaria, Moore, from 

 its present congeners when its sporangia have been discovered. 



Sporangia or sporangial frustules have been observed by the 

 writer in the following species of the genus Meloseira as now re- 

 stricted, viz. in M. varians, Ag., M. nummuloides, Ag., M. Bor- 

 reri, Grev., and in an Antarctic species collected by Dr. Hooker 

 allied to M. glohifera, Ralfs. In these species the sporangium is 

 spherical with its axis of growth corresponding with that of the 

 filament in which it is situated, and to which it continues for some 



le closely to adhere. The sporangium of M. varians, Ag., has 



lie, sometimes two projections or mammillae, each of which fits 



ito an empty half-frustule, and frequently so closely as to be 



jparablc from it. PI. XI. fig. A 1 represents various forms 



id stages of development of the sporangium of M. varians ; and 



