260 DESCEiPTioii OF \^Polygastrica. 



designate the present genus, it does not represent the beings which 

 Ehrenberg so named. The A. conrergens, (Ehrcnberg), is the only- 

 one retained in the genns by Mr. Ealfs; the A. truncatus (Ehi-.) is 

 quoted as doubtful, and all the other members are variously distiibuted, 

 one with Xanthidium, and others in Scenedesmus. 



Ehrenberg's characters of his genus Arthrodesmus, are animalcules, 

 free (no pedicle being present) ; they have a simple univalved com- 

 pressed lorica, and multiply by spontaneous seK-division, in the 

 form of tables, or compressed and articulated ribbons, each animalcule 

 being contiguous to its neighbour. Neither locomotion nor an opening 

 to the lorica has been seen. The chief animal character is self-division, 

 and their close alliance, through Micrasterias and Euastrum, to Navi- 

 cula, rather than to any known plant. The internal green colouring 

 matter of their parchment-like lorica consists of minute homogeneous 

 gi'anules resembling ova ; each cell-like lorica containing from one to 

 tliree bright bodies, analogous to fecundating glands, which are often 

 accompanied with crystalline vesicles, like polygasti-ic stomachs. 



Aktheodesmus co«fe/yms, (Ehi\) — Segments elliptic; each having 

 its spines curved towards those of the other. Erond smooth, deeply 

 constricted at the middle ; the transversely ellijitic segments (cor- 

 puscles, Ehr.) have on each side a curved spine which converges 

 with the similar one of the other segment. The frond has a gela- 

 tinous covering, sometimes distinctly seen, at other times impercep- 

 tible. (P. 2, f. 112, 113.) A. comer gens difl'ers from A. Incus, by 

 its larger size and elliptic segments. Length of ft-ond l-598th to 

 l-539th. 



A. Incus. — Minute, smooth ; end margin truncate. 



Var. fa. J — Segments externally lunate ; spines diverging. 



Yar. fh.J — Segments gibbous on each side near the base ; spines 

 of one segment parallel to, or converging with those of the other. 

 Sporangia orbicular with subulate spines. 



A. minutus, (Kiitz.) — Erond minute ; spines diverging. 



A (?) truncatus, (Ehr.) — Corpuscles gi'een, slightly compressed, 

 campanulate, geminate, externally truncate, spinous. Diameter 

 with spines 1 -480th. It is probably a Xanthidium or a Siatcrastrum. 



The two following forms have been referred, by Ehrenberg, to 

 Arthrodesnms, and though in this genus, as understood hj Mr. Ralfs, 



