Xaviculacea.'} iNFtrsoRi.vL animalcules. 329 



smaller circle, but with the tubercles rather more consj)icuous and 

 elongated, comiects the outer extremities of the rays with the extreme 

 margin of the disc. On the more elevated portions of the inflected 

 disc, the rays appear to be rather stronger than elsewhere. Diameter 

 l-145th. 



" My specimens consist of at least three layers, inclosing two 

 iinicr cavities, which contain a green endochrome. In this it 

 resembles many other allied forms. From what has appeared a 

 single disc of Arachnoidiscus Japonicus, I have scpaiatcd as mrny 

 as six siliceous layers. 



" This seiiaration into lamina, marking the existence of so many 

 distinct frustules, reminds us of Melosira and its allies ; a resem- 

 blance that becomes the more striking, when we consider, that, as in 

 Melosira, the fii'st frustules of Arachnoidiscus, Cocconeis, and many 

 others, are attached as parasites to some other body. In the analy- 

 tical table of the Baeillaria, page 221, Ehrenberg includes many of 

 these objects; classing Cocconeis, Actinocyclus, and what he calls 

 Baeillaria, together in his group of Navicul<B, and characterizing 

 them as free, in contra -distinction to his fixed forms, in which latter 

 he includes Mhnia and other genera. It appears evident, however, 

 that Cocconeis and Arachioidiscus arc las 'fixed' when found «i situ 

 as any of the Diatomacece ; and probabl)^ many of these other allied 

 genera will eventually be found to exhibit the same features when 

 better known." 



Campylobiscus Argus (Baile3^) — Large, circular, and saddle- 

 shaped, surface marked with rows of conspicuous dots ; margin 

 smooth, with a row of pinnule, placed at a short distance from its 

 edge. A fine, large, and very distinct species, which appears 

 to be widely difiused in the estuaries of the United States. 



Genus Ceeatatjltjs, (Ehr.) — Lorica bivalve, sub-cylindrical, or 

 sub-orbicular ; multiplied by perfect spontaneous fission ; hence 

 not concatenate; with two tubular apertures, and as many cirrhi, 

 alternate with them, on each surface of the lateral disc. 



This genus and Auliscus connect Zygoceros with Campylodiscus, 

 and differ in the same manner as do Biddul/phia and DenticclJa which 

 present concatenated, or imperfectly separated forms. Ccrataulus =^ 

 non-conc^tenate Denticella. 

 z 



