444 DKSt:[Ui'TI().\ oi- [Poli/gastrica. 



raities by a membrane, about 28 in the cntii'e circle. Diameter 

 1 -864th. Bermuda. 



Genus Tabellaria. — Kiitzing defines it thus: — " Bacilli adnate, 

 obsoletely stiped, at length partially separating, concatenate ; longi- 

 tudinally and interruptedly vittate ; inflated at the apices, and on 

 the secondary side." He adds, "it is distinguished from both 

 Fragilaria and Striatelh by having a large central aperture." The 

 family Tahellariece contains Tetracychis, Tabellaria Terpsinoe, and 

 Orammatophora. Tahellariece. are essentially of fresh-water habit, 

 and arc very widely distributed. They occur both living and fossil. 



In his article on Diatoma (Ann. Xat. Hist., vol. x., 1843, p. 

 .449) Mr. Ralfs, speaking of that genus, and including also Tabellaria, 

 alludes to their unattached filaments as a characteristic ; but, accord- 

 ing to Kiitzing, both these genera are obsoletely stalked. If the 

 latter be correct, both the present genus and Diatoma should have 

 found their place among the Ecuinellea. 



Mr. Ralfs further remarks — " The British species of this genus 



form two very distinct groups I believe distinct genera, 



called by Mr. Shuttleworth, Diatoma and Tabellaria. 1. Diatoma. — 

 In this no striae are seen ou the front surface, and there is no trans- 

 verse canal (vitta of Kiitzing) ; whilst the lateral surfaces have 

 transverse striae, the ends of which appear along the margins of the 

 frustviles, when these are in their usual position. 2. Tabellaria. — 

 In this group two or more longitudinal stria?, interrupted in the 

 centre by a canal, are seen on the front surface, but there are no 

 striae on the lateral surfaces." 



I'abellaria appears to be distinguished from Ehrenberg's genus 

 Tessella, by the transverse canal interrupting the striae. 



1. Lcevis=Diadesmis Icevis {YMiiz.) — Very small, smooth, length 

 five times the breadth (P. 15, f. 40). 



T. biceps. — Very small, smooth ; very turgid at the centre, late- 

 rally, ends more slender and capitate. Eossil in Ireland and in 

 North America. 



T. Gastrum. — Very small, smooth, laterally vdih. a sub-globose 

 median swelling, ends capitate, rather narrowed. Labrador. 



T. nodosa. — Small ; with five fine nodosities (nodes), of which the 



