388 ANIMAL NATURE OF DlATOMEiE. 



13. PoDOSiRA. — Individua concatenata, distindissime 

 stipitata, lorica ut in preecedenti genere. Stipes cen- 

 tralis. 



The Podosirce are merely concatenated Fyxidiculce, 

 and this concatenation results from the persistence of the 

 fine external membrane within which the deduplication 

 [sdoppiamento) takes place, precisely as in the next genus, 

 in which examples are not wanting of an uniting isthmus 

 and peduncle, by which the Melosirce also sometimes 

 become stipitate. To the two species of Kiitzing (P- 

 Itormoides, P. Montagnei,) both exotic, we can add one 

 from the Adriatic : 



Podosira adriatica : P articulis spharicis,. vix de- 

 pressis. Sig. Stalio found it in Dalmatia, parasitic upon 

 Callithamnion Borreri. The articulations are in trans- 

 verse diameter, which is the longer 0-046'", in longi- 

 tudinal diameter 004'", or a little more, and frequently 

 the two diameters are equal, and the form perfectly 

 spherical. Though not differing from P hormoides, 

 except in the lesser depression of the articulations, I still 

 consider it different because of the constancy of that 

 character. Kiitzing, to whom I sent it, suspects it to be 

 the same as in P hormoides. For my part I hesitated a 

 long time whether I should not refer it to a form of Melosira 

 monoliformis, since in the Melosircs, even, the interstitial 

 ring is at first absent; but besides that I have found its 

 absence in very numerous specimens to be absolute, I 

 found nothing that I could refer to the annular channels 

 of the Melosircs. Every articulation is constituted of two 

 valves, perfectly hemispherical, which separate on being 

 pressed; and each pair is contained in a cylindrical 

 sheath, which embraces and surrounds the contiguous 

 halves of the two articulations. This sheath has a re- 

 markable firmness, for when the articulations are -broken 

 or separated, it remains wide open and unaltered in 

 figure. It is ci'ossed transversely by fine striae, eight of 

 which are contained in 0007 5'". They are formed of as 

 many series Of minute points, projecting like papillae. It 



