380 DESCBiPTiON OF [Polygastrica. 



Typical speoies, Meloma Americana (Kiitz.), in Mr. Thwaites 

 arrangement, Orthoseira Americana, differing from the following 

 new species principally in the ends of its frustules being striated, 



ORxnosEiRA Dicldeii. — Filaments short ; cells quite smooth ; 

 other characters those of the genus ; sporangia fusiform. (P. 24, f. 29a.) 



" The filaments of this beautiful species consists generally each of 

 from two to four frustules, which are hyaline and perfectly smooth ; 

 central cavity filled with dark red-brown endochrome ; Sporangium 

 fusiform, marked with numerous annular constrictions, whose 

 formation is progressive, and which go on increasing until the 

 sporangium is frilly developed (P. 24, f. 296, a filament, the terminal 

 cells of which have each commenced to develope a Sporangium ; 

 and fig. 29c, a mature Sporangium.) This fonnation thus occui's : 

 at the commencement of the formation of a Sporangium, the endo- 

 chiome, at the same time that it withdraws from the end of the 

 frustule, produced at its centre an additional ring of cell-membrane ; 

 and this process continuing to take place at certain intervals, each 

 new ring of cell-membrane exceeding in diameter those previously 

 formed, produces at length the structure represented in fig. 29c; 

 or it may be a more correct explanation of the process to say, that 

 an entire new cell-membrane has been developed by the young 

 Sporangium at the time each new ring has been formed, and that, 

 thus have originated the several chambers into which the ends of 

 the Sporangium are divided ; fissiparous division subsequently takes 

 place, and sporangial frustules are developed from each half, as 

 shown in fig. 29d. This sjiecies was found by Dr. Dichie, in 

 a dark dripping cave close by the sea, near Aberdeen, and covering 

 the mosses, Hepaticae, as a fine blackish green sand. (.\nn. Nat. 

 Hist. 1848.) 



Genus GoNioTHEcnm (Ehr.) — Lorica round, not in chains, having 

 a central consti'iction or furrow, each end abniptly attenuate and 

 truncate, so as to assume an angular figure. 



This genus resembles Pyxidicula, but has a central consti'iction and 

 truncate ends. Fossil. 



G. Roger sii. — Testules in pairs, (binate), smooth ; each dorsally 

 sub-quadrate, angular with three vcrticilli ; laterally, elliptic oblong, 



