DONKIN, ON DIATOMACEiE. 6 



beach, arranged in distinct zones, in the lower of which 

 (that near the low water margin) the Toxonideee, PI. lanceo- 

 latum, PI. falcatum, N. lyra, N. forcipata, &c., occur 

 abundantly. In the upper zone, or that near high- water 

 mark, the predominant forms are Cocconeis excentrica, N. 

 palpebralis, Amphiprora pusilla, Ep. marina, Nitz. virgata, 

 Nitz. spathulata, &c. ; in the middle zone, A. arenaria, N. 

 granulata, N. humerosa, N. Clepsydra, N. Northumbrica, N. 

 truncata, &c., are abundant. 



Before proceeding to describe the new species which I am 

 about to introduce, I consider it necessary to make a few 

 observations in defence of some of those already published 

 in my previous contribution. Professor Walker Arnott has 

 asserted that the two species forming my new genus Toxo- 

 nidea, T. Gregoriana and T. insignis, are mere twisted or 

 distorted conditions of Pleurosigmata, the former of PL 

 angvJatum, the latter of PI. cestuarii or PL lanceolatum. 

 Dr. Arnott observes,"^ " In Pleurosigma I have seen no 

 instance in which the living frustule is twisted" ^ * ik a i\^q. 

 S.V. is sigmoid with the median line nearly equidistant 

 from the two sides ; but after the valves are detached from 

 the connecting zone, they often become slightly twisted, and 

 as they cannot then present a flat surface to the eye, the me- 

 dian line appears to approach nearer to the one margin than 

 the other ."^ This is a confession and explanation of views, on 

 the part of Dr. Arnott, in itself fatal to his hypothesis ; for 

 it shows that the twisting or distortion, which he avers the 

 ToxonldecB have been subjected to, is not a vital change 

 found in the living frustule, but is the result of boiling in 

 acid, and of drying the valve on glass slides. To prove the 

 inaccuracy of this assertion, I have only to observe, that I 

 have examined numberless specimens of both Tox. Grego- 

 riana and insignis in a living state, moving in their native 

 element ; and that the shape of the valve, and the relative 

 position of all its parts, in each species, is exactly that repre- 

 sented by me in my descriptionf and figures of them. To 

 my own testimony I may be allowed to add that of my 

 friend. Dr. Gre\ille, to whom I sent a living gathering, 

 abounding in these species. In nearly all the very numerous 

 gatherings I have, from time to time, made on the Nor- 

 thumbrian shore during the last four summers, I have found 

 the Toxonideae in some localities in great abundance, and in 

 all they preserve a remarkable uniformity of contour and 

 markings. PL angulatum, on the contrary, is not a shore 



* 'Micr. Journal,' vol. vi, p. 199. 



t ' Trans. Micr. Soc. Loud.,' vol. vi, p. 19, new series. 



