Naviculacea.'] inftjsoijial animalcules. 401 



The difficulty is illustrated by the fact, that of the forty-live 

 species of Nmicula, described after Ehrenberg, in the former edition 

 of this work, ten have been since transferred, by that most accurate 

 observer of species, to other distinct genera, apart from the many 

 re-distributions he has made among the several sub-genera. 



Kiitzing separates several species from Kavicida, by reason of their 

 being symmetrical ; that genus being peculiar in always having a 

 symmetrical lorica, — i. e., one equally developed on each side the 

 central umbilicus. Those species so removed, arc included in the 

 genus CymleJla. 



Sub-genus Sijiiirella. — Striated ; no umbilicus ; in both these 

 circumstances difiering from Kavicida; and, in the latter of the 

 two, from Pinnularia. The following additional characters and 

 remarks are from the recent valuable contribution in the Annals of 

 Natural History, by the Rev. W. Smith. (1851, p. 7.) " Valves 

 concave, with a longitudinal central line, and margins produced 

 beyond the suture (winged.) Friastules free, solitary, or, when under- 

 going self-di'vision, in pairs- The concavity of the valve, their 

 winged margins, and the longitudinal central line, which wants the 

 central depression (umbilicus.), so conspicuous in the Naviculece, are 

 characters which sufficiently distinguish Surirella from all other 

 genera." 



Mr. Smith has detected alse in six species (S. hiseriata, splendtda, 

 gemma, fastiwsa, craticula), and he believes them present in all. It is 

 only on an end view of the valves, but rarely to be obtained, that the 

 alse can be clearly seen. 



"The costae, so conspicuous in several species, as well as in Campy- 

 lodiscus costatus and spiralis, appear to be caxised by canals or tubes 

 passing between the siliceous valves and the inner membrane of the 

 cell ; these canals commvuiicate with the exterior by a series of 

 perforations, along the suture or line where the connecting membrane 

 imites Avith the valves. Accepting the Diatom as a vegetable 

 organism, these tubes will be regarded as analogous to the inter- 

 cellular passages, and the exterior perforations will perfonn the 

 office of the stomates of the leaf. (At most, these perforations can 

 have but a remote analogy with stomates, for these organs are peculiar 



