238 DESCEiPTiON OF {Polygastrica. 



animal combustion, are other facts respecting this family, which, taken 

 singly, might have less value, but in their combination furnish a 

 most important support to the arguments already adduced. 



" The DesmidiecB I regard then as Algai, allied on the one side to 

 Conjugated, by similarity of reproduction, and on the other to the 

 PalmelUcB, by the usually complete transverse division, and by the 

 presence of gelatine. Indeed, the relation to the latter is so intimate, 

 that it is difficult to say to which family some genera belong ; — Some 

 species of Scenedesmus may be allowed to have an almost equal claim 

 to rank with either." 



An additional exemplification of the affinity of the Desmidiece with 

 the Algae, has been recently afforded by detection by Mr. Jenner, 

 confirmed by Mr. Ealfs, in a species of Tyndaridea, of evident, 

 though faint, longitudinal striae, similar to those in many Closteria. 

 Mr. Bowerbank has also pointed out the same appearance in 

 Tiresias. 



Owing no doubt to the soft, or but slightly siliceous integument of 

 the Desmidiece, hut few forms have been found in a fossU state, 

 among such are some species of Closterium and Euastnim, and 

 doubtful specimens of Xantkidium. 



In Ehrenberg's system the Desmidiece formed one division of his 

 family Bacillaria, the characters of which, are given in the table, at 

 page 221. 



The following genera have been established since this table ap- 

 peared: namely — Lithodesmium, Eucampia, Asterodictyon, Monactinus, 

 Gymno%yga, Dyalotheca, Polysolenia, and Zygoxanthium, The genus 

 Hyalotheca is adopted by Mr. Ealfs ; of Eucampia and Lithodesmium 

 we have figures ; but the other genera are known to us only by 

 name, or by imperfect descriptions. 



Genus Hyalotheca, (Ehr.) — Filaments elongated, cylindrical, 

 very gelatinous; joints having either a slight constriction, which 

 produces a crenate appearance, or a grooved rim at one end which 

 forms a bifid projection on each side. End view circular. 



The filaments are invested with a broad gelatinous sheath ; are 

 very fragile in one species, but not so in the other. The cyhndrical 

 filament distinguishes this genus from Desmidium and 8ph(tro%osma. 

 It has no angular projections, is not twisted, and has always the 



