ANIMAL NATURE OF DIATOME^l. 409 



Adriatic specimens by their larger curvature and more 

 decided transverse striae. 



The family of Achnantheae is also distinguished from 

 all others by the complicated structure of the shield. 

 The primary surfaces, Kiitzing says, are formed of three 

 pieces, two lateral, transversely striated, and one median 

 traversed by two longitudinal striae with terminal per- 

 forations corresponding to their extremities. Hence 

 every individual will appear to be formed of eight valves. 

 To me it appears, on the contrary, that these transversely 

 striated lateral portions can by no means be distin- 

 guished from the secondary surfaces ; there being neither 

 angle nor joint to indicate the supposed distinction. 

 I do indeed find that the two halves of each lateral 

 surface are inclined to each other like a roof, and they 

 easily become detached one from the other, thus con- 

 stituting, together with the two primary surfaces, at 

 least in appearance, six valves. The internal funnel, 

 shaped appendage which accompanies the central per. 

 foration of the inferior valve, is really very remarkable. 



24. CYMBELLA. Individua solitaria vel geminata^ 

 libera (nee adnata nee indusd), curvata incequilatera ; 

 latere primario uno (interiori ventrali) artgustiore, altero 

 (exteriore, dorsali,} latiore; laleribus secundariis cequalibus 

 (transversim striatis} ; aperturis mediis marginalibus ap- 

 proximatis. 



In general form, in the parellelism of the curved 

 primary surfaces, in the inclination of the secondary 

 surfaces, in the trapezoidal transverse section, and in the 

 mode of attachment when parasitic, (C. Pediculus,} the 

 Cymbellce are very similar to the fipitkemite. They 

 differ essentially from these by the two perforations 

 placed in the border of the inferior side of the lateral 

 surfaces, and therefore sufficiently near each other to 

 seem united into one when placed obliquely. They have, 

 moreover, a distinct aperture at each extremity. The 

 difficulty of making out these characteristic perforations 



