400 ANIMAL NATURE OF DIATOMEiE. 



continued or interrupted, to be placed among the Den- 

 ticulcB or the Surirellce. In one {S. quadranguld) we 

 have the character of Denticulee, the excess of the pri- 

 mary surface over the secondary; in others {S. virc/inalis, 

 S. constrida,) we have the median contraction of the 

 primary surfaces characteristic of the second section of 

 Surirellce. Thus, indeed, all these species only want the 

 central aperture to belong to the Naviculce or Ach- 

 nantJiidia {S. Biasollettiana). Certainly we cannot say 

 that the presence or absence of these characters is of 

 little importance ; we have only to remark that with the 

 highest powers of the microscope it is often impossible to 

 decide with certainty whether they have the transverse 

 striae and the median aperture or not. 



** Echinaria : Icevissimts, demum affixcB et plerumque 

 radiatim aggregatce. 



The seventeen species contained in this section do not 

 differ materially from those in the former, except in being 

 affixed. Many of them are attenuated at the extremities 

 of their primary surfaces ; but those destitute of that 

 character {8. ampJdcephala, S. tenuissima, S. tenuis,) 

 have great analogy in form with the TJlnaria. We have 

 also species in this section that are curved in their 

 secondary surfaces {S. curvida, S. Arcus,) and in their 

 primary ones, {8. lunaris, 8. bilunaris,) which resemble 

 the Eunotice and Achnanthidia. The 8. amphicephala 

 is distinguished from all by its dilated extremities. 



*** Ulnaria : ajjixce, flabellatim disrupfce, in latere 

 secundaria, excepjto spatio medio longitudinali lesvi, trans- 

 versim striata. 



Although the twenty-four species of this section have 

 nothing in common but the general character of all the 

 8gnedrce (their bacillary form and the absence of a 

 median aperture), still we find enumerated among them 

 species that are unattached, {armoricana, sigmoidea, 

 vermicularis,) and even destitute of the characteristic strife 

 {vermicularis) ; and this because the spirit of system (5/?f;7Vo 



