398 M. Ehrenberg on the Living Representatives of the 



Oran, which M. Ehrenberg had considered as but little dis- 

 tinct in a species of the genus Navicula, which was distin- 

 guished by a kind of internal folds, and which he has de- 

 scribed in his preceding communications under the name of 

 A~. Africana, considering it at the same time as character- 

 istic of the chalk-formation. 



The examination of the living form has shewn that this 

 Navicula, which seemed to deviate widely from the genera 

 now existing, is related to one nearly allied to those now liv- 

 ing, and in which the distinct acieuli, in consequence of an 

 imperfect division, present zig-zag bands. The three cham- 

 bers in the living animalcula are occupied by three greenish 

 corneous cavities or discs which have been regarded as the 

 ovaries in all the family. The middle of the body, as in Ta- 

 bellaria, is occupied by a hollow transverse tube, the openings 

 of which appear in every respect similar to the two medial 

 openings of Navicula ; but their functions do not appear to be 

 the same, since these openings are not free, but rather fitted 

 closely to the similar openings in the allied animal. 



This imperfect distinction of the individuals, the develop- 

 ment in the shape of a ribbon, as well as the necessarily dif- 

 ferent position of the alimentary openings in the forms of the 

 marl when compared with the Navicular, completely separate 

 them from these, in like manner as the subdivision of the in- 

 terior chambers removes them from the Tabellariae. They 

 thus form a particular generic group, which is completely dis- 

 tinct from European forms. This second new genus has been 

 named Grammatophora, and the species G. oceanica. 



The apparent identity of the fossil N. Africana with the 

 Grammatophora oceanica extends only to the genus and not to 

 the species, since an alga found at Vera Cruz by M. Carl 

 Ehrenberg, and which has quite recently come to hand, has 

 furnished two other forms very nearly allied to this genus, 

 which have been distinguished by the names G. Mexicana and 

 G. undulata. The partitions in these forms often present the 

 figure of a point of interrogation. 



An example of Coscino-discus excentricus has likewise been 

 found on a Mexican alga, and there, as at Cuxhaven, it lives 

 in the sea, and its fossil coverings have also been discovered 

 in the chalk-marl of Oran in Africa. 





