Stodder, on Diatomaceee. 27 



recall Dr. Gregory's query, whether they should not aU be 

 considered one species. The same observations apply to 

 Naviculce of the type of A^. lyra. 



There are two forms of Ehrenberg's genus Actinocyclus, 

 called bymost 2iK\.i\\ox\i\e&Eupodiscus ; one ve^e.mbAe'&E.sparsus, 

 Greg, E. tenellus, Breb., and the Actinocyclus of Ehrenberg 

 (^Mic. GeoL' Taf, xix, fig, 5, c. 10). Also Coscinodiscus lunce 

 (Tab, 35a, group xxi, fig. 7) ; Cos. gemmifer (Tab. 35a, group 

 xxii, fig. 3) , This form is distinguished by rays composed of 

 lines of contiguous dots, with other dots irregularly scattered 

 between the rays. The number of rays is very variable, from 

 six upwards ; sometimes the rays are so crowded, that the inter- 

 mediate dots almost form continuous rays, only distinguish- 

 able by their irregular distance from each other; colour, 

 usually some shade of brown. 



The other form of Actinocyclus has very fine lines for 



rays, not always continuous ; and the whole surface of the 



disc is covered with a very fine network of, probably, hexagonal 



markings, too fine to be well made out with my instrument. 



This form is represented by Eupodiscus fulvus, W. Sm., and 



possibly by E. subtitis, Ralfs ; by a great many of Ehrenberg's 



species, 'Mic. Geol.,^ Tab. xviii, fig. 8, c. 18, Richmond, 



„ „ xxviii, gr. 22, fig. 7, 



„ „ xxxv,A, gr. 17, fig. 1, and 2, guano, 



„ „ „ gr.l8, fig. 1,2, and 3, guano, 



Saldauha Bay ; 

 also Strafford Cliffs and Eappahanock Clifts, var. colour, 

 usually blue or purple, sometimes brown, and sometimes 

 colourless. Both of these forms have generally, but not 

 always, a nodule or process near the margin, resembling the 

 " feet " of Eupodiscus and Aulacodiscus ; which is probably 

 the reason of their having been taken for Eupodisci, though 

 the structure of the valve appears entirely different from the 

 true species of that genus, Ehrenberg does not figure or 

 describe the nodule, but on examining the Actinocycli of 

 Saldanha Bay, in the Bailey collection, received, I believe, by 

 Bailey from Ehrenberg, I find the nodule is present in them, 



Ehrenberg' s figures are sufficient to indicate the genus, 

 but not the species, except by the number of the rays, which 

 is not a good specific character, neither is colour. But I am 

 Avell satisfied that many of the so-called Eupodisci are 

 Ehrenberg's Actinocyclus ; in fact, it is almost admitted by 

 Smith. 



These two forms of Actinocyclus should probably be placed 

 in two genera. They have quite a different structure ; that 

 of the first- mentioned is not cellular, but the dots are pro- 



