816 



SYSTEMATIC HISTOEY OF THE INFUSORIA. 



species are numerous, and sometimes differ veiy slightly ; we fear indeed that 

 several of them have been constituted upon insufficient grounds. Attempts, 

 more or less successful, have been made to remove some species, and to form 

 wdth them new genera. We have used these divisions as sections, partly 

 because we are unable to find at present differential characters sufficient to 

 justify a more complete separation, and partly because we cannot decide 

 absolutely on the proper position of several species. The following are the 

 sections we have adopted : — 



* Lijs'ujonium. — Joints globose or elliptic, with a ring-like keel round each 

 valve. In this section the fnistules resemble in form those of the two pre- 

 ceding genera, but are distinguished by their carinated valves. The suture 

 is a ridge between the valves. 



2 * GallioneUa. — Joints cylindrical or suborbicular, with a single median 

 furrow, and more or less rounded ends, generally binately connected ; valves 

 not carinated. The filaments are more or less internipted at their margins, 

 and the junction-siu'faces are not denticulated. 



3 * Aidacosira. — Joints cylindiical, bisulcate, with more or less rounded 

 extremities. The genus Aulacosira was proposed by Mr. Thwaites for 

 " those species characterized by the absence in the frustule of an evident 

 central line indicating the place of subsequent fissiparous division, but each 

 frustule having two somewhat distant sulci or fossulte passing round it." We 

 have found the sulci constant; but Professor Smith believes "the characters 

 have no real existence, and owe their apparent presence in the species Mr. 

 Thwaites adopted as his type, \iz. Mehsira crenulata, Kiitz., only to accident, 

 or observation under imperfect illumination. A careful study of the specimens 

 from Aberdeen, upon which Mr. Thwaites himself founded his remarks, and 

 of gatherings from various other localities, has failed to satisfy " him " that any 

 essential differences exist between this species and other Orthosirse." 



4 * Orthosira. — Joints exactly cylindrical, marked by a central line, con- 

 nected into an uninterrupted cylindrical filament; internal cavity often 

 spherical or subspherical (Thwaites). Orthosira contains " those species 

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

 therefore form, by their close contact, an uninterrupted cylindiical filament " 

 (Thwaites). Professor Smith distributes the species of Melosira under two 

 genera, Melosira and Orthosira, which he thus defines. Melosira : " Fila- 

 ments cylindrical, of numerous frustules, attached or free ; frustules spherical 

 or subcylindiical, more or less convex at the junction-surfaces." Orthosira : 

 ^' Filaments cylindrical, of numerous frustules, continuous, attached or free ; 

 frustules and valves cylindiical ; junction- surfaces plain, hue of junction 

 usually spinous or denticulated." We regret that in the present state of our 

 knowledge we cannot adopt Orthosira as a substantive genus. Its junction- 

 margins, indeed, are usually denticulate or spinous, a character we do not find 

 in Melosira as defined by Professor Smith ; but this character is not considered 

 essential. In our opinion, too, there is greater affinity between " Melosira 

 distans'' and '' Orthosira orichcdcea " than between the latter and '' Orthosira 

 sulcata^ Melosira is of peculiar interest, as it affords the most frequent 

 examples of that form of reproduction in which the valves of a fi'ustule sepa- 

 rate, and a sporangium is formed between them, unattended by conjugation. 



* Frustules encircled tvith median and 

 lateral ridges. Lysigonium. 



Melosira nummuloides (Dillw^n, 

 Ag.). — Frustules spherical, very finely 

 punctured ; valves carinated. ^^^ 



Sbd. 



p. 56, pi. 49. f. 329. =M. discigera, Ag. 

 Marine and brackish waters. Eui'ope, 

 America, (v. 64. xi. 14.) Diam. 1-1700" 

 to 1-860". Frustules globular, united 

 in pairs, forming a moniliform filament ; 

 each divided into hemispheres by a cen- 



