OF THE PEDIASTEE^. 



'53 



referred by jS'ageli, or, with some few other Algte, form a distinct group near 

 Pahnellaceae, and perhaps Volvocinese. They cannot, we think, continue 

 to be considered as belonging to the Desmidiaceae. For the pui-poses of the 

 present work, however, as they are introduced, we shall just indicate that 

 the genera here described under the above head agree in the following 

 characters : — 



Cells combined into a definitely formed frond or family, often cither ex- 

 ternally notched or attenuated, sometimes spinous, not undergoing complete 

 self-fission in the same direction into two perfect cells, but propagating by 

 the repeated segmentation of the contents of the old cells into a definite 

 number of portions or " gonidia," which are either still or for a time motile, 

 and which are either arranged according to the typical plan within the parent- 

 cell, and by its bursting set free as a new frond or family, or become so 

 arranged without the parent-cell, but still involved in its inner membrane, 

 the whole ha\'ing emerged by a transverse fissure. 



We are disposed to think that here Hydrodictyon should come ; for though 

 in this plant the development of the active gonidia is simultaneous, not 

 successional, as in Pediastrium, Pringsljeim alludes to the gonidia in Ccelas- 

 trwn sphcencum (which indeed are still) as either the one or the other. 

 Cruciagema quadrata (MoiTcn) =Staurogenia quadmta (Kg.), seems to pro- 

 pagate by complete self-fission, and, gonidia not being described, we believe 

 cannot belong here. As to Sphaerodesmus (i^iig.) information is wanting. 



Genus SCEXODESMUS (Meyen).— Frond or family composed of from two 

 to eight oblong fusiform or elliptic cells, mnnected into a single 07^ double 

 continuous row ; propagating by means of the repeated segmentation, in 

 parallel planes in one or two dii-ections, of each of the cell- contents into one 

 or more brood families (not motile), set free by the bursting of the parent- 

 ceU wall. (I^ag.) 



ScENODESMUS qundricauda (Ralfs). — 

 Cells in a single low ; oblong, rounded at 

 then* ends ; external cells (sometimes 

 more turgid than the others) furnished 

 at each extremity icith an elongate^ often 

 curved, acute spine or bristle, sometimes 

 with another from the centre of the 

 outer margin. L. 1-1121" ; B. 1-2631". 

 (i. 40, 41, 4S.)=Achnanthes quadricauda 

 (Tui-p.), ArtJirodesmus quadricaudatus 

 (Ehr.), Scenedesmus caudatus (Corda, 

 Kg.), S. quadricaudatus (Hass.). G.B., 

 I., F., G., U.S.A. /3, central cells fur- 

 nished at one of their ends with an 

 elongate, acute, curved spine or bristle, 

 each half of the frond being so fm-nished 

 at opposite sides, sometimes the central 

 cells being also fm-uished at their other 

 ends watli a verv short, minute spine. 

 = .S'. A%e/«(Breb.). (i. 42.) 

 . S. dispar (Breb.). — Cells two or four, 

 alternating, oblong, bhmt at the ends ; 

 when fom* the central cells at one end at 

 opposite sides of the frond furnished %x\th 

 a short acute mucro-like dejected spine, 

 each spine directed inwards ; when either 

 two or four, the external cells with a 

 similar spine at both ends; when four, 



I that spine at the same side of the frond 

 I with that belonging to the central cells 

 I also directed inwards, the other directed 

 j outwards. F. 



I S. antennatus (Breb.). — Cells in a 

 I single or double row ; fusiform, or semi- 

 lunate, ends cuspidate, and each tefrmincded 

 by a minute orbicular globule. F. 



S. dimorphus (Kg.). — Cells in a single 

 row ; narrow, attenuated, and pointed at 

 the ends; the central in apposition the 

 most of their length, the outer externally 

 lunate. L. 1-1020" to 1-906"; B. 1-8160". 

 = Achnnnthes dimorpha (Turp,), S. pcc- 

 tinatus (Meven), ArtJirodesmus pectinatus 

 (Ehr.). G.B., I., F., G. 



S. acutiis (Meyen). — Cells in alter- 

 nating rows; the central fusiform, in 

 app)osition only at their middle, the outer 

 sometimes extemallv lunate. L. 1-1663" 

 to 1-1060"; B. l-62o0" to 1-6181". = 

 Arthrodesmns acutus (Ehr.), S. acutus et 

 oUiquus (Ralfs). G.B., I., F., G., Italy. 

 S. obtusus (Meyen). — Cells in one or 

 oblong, end^ 



two rows, all ovate 

 rounded. L. 1-2331" 

 1-4096" to 1-3623". 

 G.B., L, F., G., U.S.A 



or 



to 1-1961' 



(i. 37, 38, 



3c 



; B. 



39.) 



