390 DESCRIPTION OF {Volygaslrica. 



M. sexangula. — One of the thin valves \vith six angles and rays, 

 the other not known ; apex broadly truncate, presenting an hexa- 

 gonal area. Diameter 1-1 632nd Bermuda. 



All the above species are very smooth and crystalline. 



M. heptagona. — One valve with seven, the other with nine rays 

 and angles, and a truncate apex. Diameter 1 -840th. Bermuda. 



M. Actinoptychus = Pyxidicula Actinoptgclms. — One valve with 

 nine angles and rays, the other with thirteen. Apex broadly trun- 

 cate, smooth. Rays loose and flexuose. Yirginia. 



M. Oculus Chameleontis = Pyxidicula 0. C. — One valve with eight 

 angles and rays, the other unknown. Apices truncate. Diameter 

 1-11 52nd. Maryland. 



Besides the above forms, single valves are met with in earth from 

 Bermuda, having fifteen to seventeen and nineteen rays ; and, in 

 earth from Vu-ginia, with thii-teen, fifteen, nineteen, and twenty 

 rays ; and such may constitute other species. 



M. Discoplea. — Small, valves conical, ti'uncate; margin and area 

 truncate, with smooth apices ; eighteen to twenty rays and angles. 

 Diameter 1-1 152nd. The variety faj with eighteen rays. More 

 common, in the pumice of Patagonia ; var. (bj with twenty, more 

 rare, in the same substance. 



Genus Mekidost. — The characters of this genus agree, generally, 

 with those of Fragilaria, but the frustules being cuneate (wedge 

 shaped) form, when concatenated by imperfect self-di\asion, circular 

 or spii'al bands. It is closely allied to Navicula, but differs by its 

 incomplete self-fission, and its frustules have no central aperture, 

 but two placed at their wider extremity ; single wands (frustules) 

 are difficult to distinguish from Gomphonema, but the latter possesses, 

 besides two terminal pores, a central one also. 



Kiitzing defines this genus, the type of his family Meridie^y 

 thus: — 



" Individuals cuneiform, rectangular, prismatic ; closely conjoined 

 in flabelliform masses or in spiral bands. Transverse striae strong, 

 uninteiTupted." 



The following remarks are from Mr. Ealfs valuable papers (Ann. 

 Nat. Hist., 1845) : " This genus, together with Styllaria, Gompho- 



