INTRODUCTION TO CRYPTOGAMIC BOTANY. 129 



the first of which contains those with smooth or transversely 

 striate frustules, but neither vittate nor areolate, the second with 

 vittate and the third with areolate frustules. The first compre- 

 hends far the larger number of species and the least interesting 

 forms. Some are perfectly free, as Navicula, others attached by 

 one surface, as Cocconeis; some form long threads, the frustvdes 

 of which adhere tolerably firmly at their commissures, as Fra- 

 gillaria, or separate alternately, being suspended at a single 

 point so as to form curious chains, as Diatoma; a few, in conse- 

 quence of assuming a circular disposition, are cuneate, as Meri • 

 (lion ; others approximate some species of Confervce, forming 

 straight lines with more or less cylindrical frustules, as Melo- 

 seira ; some, again, have singularly curved discs, as Surlrella 

 campylodiscus ; one group has strongly developed, and some- 

 times branched stems, as Gomphonema ; one curious set is 

 always surrounded by copious mucilage, so as to form distinct, 

 and often branched fronds, as Schizonertia ; while Dickiea 

 takes the habit of an UKn. They are produced both in fresh 

 and salt water, and even those genera like Schizonema, which 

 were once supposed peculiar to the sea, have their fresh water 

 representatives.* • 



102. We now come to the vittate species, which are known 

 by the fillet-shaped markings on their disc. The species are 

 far less numerous than in the last group, and more common in 

 salt than in fresh water. They grow on Algas or other marine 

 substances. Many, as Licmophora, exhibit beautiful fan-shaped 

 groups, seated on gelatinous branched threads, the tips of which 

 are dilated to bear the frustules. Licmophoraflahellatais one 

 of the finest microscopic ornaments of our coasts, but it is very 

 easily injured. Climacosphenia, remarkable for the moniliform 

 marking on its disc, has at present occurred only in New Hol- 

 land and Mexico ; and Terpsinoe in the latter country and 

 tropical America. Several other genera occur on our coasts, 

 as Striatella, and one or two beautiful allied objects in fresh 

 water, as Tetracyclus lacustris, and Tahellaria jiocculosa. 



* The genus Schizonema is not confined to the northern hemisphei'e. 

 Schizonema crispum, Mont., is found in Lord Auckland's gi-oup and 

 elsewhere, attached to the fronds of the smaller marine Algas. 

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