432 BESCEiPTioN OF {^Polygastrica' 



doubtful. Its inner wall is occupied by filiform rings, i. e., it is 

 annular. 



This genus is in form allied to Aulisms, but if concatenate, ap- 

 proacbes rather to GalUoneUa. In habit it is nearest to Liparogyra. 

 It is remarkable by the marginal circle of apertures or pits. 



PoKOCTCLiA dendrophila. — Elongate, cylindrical, the length double 

 the width, smooth, with internal annual gyroe, nine in 1 -324th ; mar- 

 ginal pores twelve in number ; the disc with very minute rows of 

 dots, and with five central apiculi. Length l-324th. "Width l-576th. 



Genus Peoeostaurtjs (Ehr.) — ^Has the characters of Stauroptera, 

 except that its pair of terminal apertures are not marginal, but 

 median (approximated in the median line.) 



This may be considered a sub-genus of Navicula ; it has been 

 instituted by Elu'enberg, but we have not met with his account of 

 the species. 



Genus Pyxidicula. (E1u\) TJie rottnd-hox Bacillaria. — Lorica 

 simple, bivalve, of a globose form, and marked by a fiuTow, along 

 which it easily separates into two hemispheres. They are never 

 concatenate or clustered. In organization this genus is closely allied 

 to GallioneUa, but difi'ers in not forming chains like it. The contents 

 of the lorica arc of a greenish-yellow colour. 



In 1836, Ehrenberg discovered, in the flints of the neighbourhood 

 of Berlin, numerous sj^hcrical bodies, of pretty equal size, their 

 diameter varjing from 1 -240th. to 1-1 150th. These, he considered, 

 probably belonged to the siliceous Biatomecc ; they occurred along 

 with siliceous spicula of sponges, species of Xanthidia and of Peri- 

 dinium ; whether they shoidd be placed in the genus Fijxidicula is 

 doubtful, as no fun-ow or division was observed in their shell. 

 (See Plate 12.) 



Kiitzing, after having examined these bodies, has come to the 

 conclusion that Ihey cannot be reckoned with the Pyxidicula. This 

 author admits Pyxidicula into the flimily Melosirece, and thus cha- 

 racterizes it : — 



" Individuals solitaiy, or conjoined in pairs, free, or sessile; primary 

 side obsolete (absent), secondary side convex (lorica bivalve, valves 

 coavcx, ^^ ithout interstitial rings.)" 



