DIATOM ACE.*: 



607 



liculi ' are most developed, and it is consequently here that they mav 

 be best studied ; and of there being here really costce, or internally 

 projecting ribs, no reasonable doubt can remain after examination 

 of them under the binocular microscope, especially with the ' black - 

 ground ' illumination. The form of the valves in most of the spec-it -s 

 is circular or nearly so ; some are nearly flat, whilst in others the 

 twist is greater than in the species here represented. Some of the 

 species are marine, whilst others occur in fresh water ; a very 

 beautiful form, the C. clypeus, exists in such abundance in the 

 infusorial stratum discovered by Ehrenberg at Soos, near Ezer. in 

 Bohemia, that the earth seems almost entirely composed of it. 



The next family, the /Striatellece, forms a very distinct group, 

 differentiated from every other by having longitudinal costse on the 

 connecting portions of the frustules, these costse being formed by 

 the inward projection of annular siliceous plates (which do not. 

 however, reach to the centre), so as to form septa dividing the cavity 

 of the cell into imperfectly separated chambers. In some instances 

 these annular septa are only formed during the production of the 



FKJ. 454. Cainpi/lodisc-us costatus : A, front view; B, side view. 



valves in the act of division, and on each repetition of such produc- 

 tion, being thus always definite in number ; whilst in other cases 

 the formation of the septa is continued after the production of the 

 valves, and is repeated an uncertain number of times before the 

 recurrence of a new valve-production, so that the annuli are indcjin //< 

 in number. In the curious Grammatophora serpentina (fig. 4f>2) 

 the septa have several undulations and incurved ends, so as to form 

 serpentine curves, the number of which seems to vary with the 

 length of the frustule. The lateral surfaces of the valves in Gram 

 inatophora .are very finely striated, and some species, as (/'. subtilissima 

 and (T. iarina, are used as test-objects. The frustules in most of 

 the genera of this family separate into zigzag chains, as in Dtatoma ; 

 but in a few instances they cohere into a filament, and still more 

 rarely they are furnished with a stipe. The small family Terpsinoece 

 was separated by Mr. Ralfs from the Striatellece, with which it is 

 nearly allied in general characters, because its septa (which in the 

 latter are longitudinal and divide the central portions into chambers) 

 are transverse, and are confined to the lateral portions of the 



