482 ANIMAL NATURE OF DIATOME^. 



lorica bivalvis discoidea circularis, in ntroque latere 

 {secundario) tribus processihus appendiculaia. 



The only species of this genus, which of itself consti- 

 tutes the family of Tripodiscesp, does not essentially differ 

 from the Coscinodisci, except by three short appendices 

 projecting from each of the lateral surfaces, which are 

 tubular, and terminate in an aperture. Ehrenberg, 

 also, suspects the presence of other smaller marginal 

 apertures. The areolation of the surface is so confusedly 

 represented in the figure, as to lead to the belief that 

 even the organic condition is complicated, which gives 

 that appearance. 



65. IsTHMiA. — Individua trapezoidea vel rhomboidea, 

 compressa, celMosa, zona transversali ex cellulis minoribus 

 formata, notata, stipitata, isthnis majoribus in catenas 

 subramosas irregulares conjuncta. Divisio oblique trans- 

 versa. 



The complicated synonymy of the two species of this 

 geniis {enervis, nervosa) justifies the new name given by 

 Kiitzing to the second. The Conferva obliquata of Smith 

 belongs to the first, and therefore Ehrenberg was in 

 error when he bestowed upon it a new specific name 

 {enervis). The other required to be named, and Kiitzing 

 was fully entitled to call it L. nervosa. By the laws of 

 synonymy, therefore, the two names obliquata and 

 nervosa ought to remain. The longitudinal costae of this 

 latter are quite extraneous to the surface ; they project 

 slightly into the interior cavity. Both in the one and 

 the other the siliceous cellular membrane of the median 

 portion persists, whilst within it a formation of two 

 lateral valves takes place, which complete the two indi- 

 viduals preceding from the deduplication. This process 

 reminds us of that of the Achnanthese, and approaches 

 the reduplication of Desmidiese. 



The transverse zone, represented by Kiitzing in the me- 

 dian portion, is neither constant nor regular. I believe it 

 to be produced by the permanence of some portion of the 



