8 DONKIN, ON DIATOMACEiE. 



tule linear on S, V. ; on F. V. falcate, or gently arcuate. V. 

 pale straw colour, on S. V._, narrow, linear, slightly sigmoid; 

 extremities rounded ; median line strongly sigmoid ; on F. V. 

 twisted laterally and falcate. Length from "OOGO" to "0070", 

 breadth of S. V. about -0006" ; striae oblique, fine. 



The peculiar form of this singular species is owing to the 

 entire frustule being twisted laterally on its long axis, and to 

 its being curved in the form of an arc. The frustule has, 

 therefore, one valve curved forward, and convex on its outer 

 surface ; the other bent backwards, and concave in its outer 

 surface. The peculiar lateral twisting of the valve is well 

 seen in its F. V. (fig. 1, c). 



When examined in the living state, this species has all the 

 appearance of a Toxonidea, between which genus and the 

 Pleurosigrnata it forms a connecting link ; it is, however, a 

 genuine Pleurosigma, in which the twisting and curv^ature of 

 the frustule are natural and not accidental conditions. To 

 examine the entire frustule in a prepared state, the material 

 must be macerated in alcohol and ether, and afterwards roasted 

 on a thin glass cover. 



Hab. Cresswell and Boulmar Bay ; plentiful, June to Sep- 

 tember, 1858 and 1859. 



2. Navicula Trevelyana* n. sp. (fig. 2). — Form on F. V. 

 elongated quadrangular, constricted laterally ; on S. Y. linear, 

 extremities rounded, margins slightly bulging out near the 

 extremities and middle ; valve exceedingly convex, inflated, 

 with large orbicular unstriated space around central nodide ; 

 median line curved ; striae coarse, costate, strongly convergent 

 around central nodule, strongly divergent near extremities. 

 Length, from -0040" to '0050"; breadth of S. V. about -0008". 



This beautiful species I have found in gatherings viith 

 N. rectangulata, Greg., to which it is closely allied, but twice 

 as large, and widely different in specific characters. 



Hub. Cresswell and Duridge Bay. May, June, and July, 

 1857, 1858, and 1859. (s, < ^ ' 



3. Navicula clepsydra,^ n. sp. — Form on F. V. elongated 



* Dedicated to Sir Walter Calverley Trevelyan, Bart., Wallington, 

 Northumberland. 



f I have placed this species, as well as the new species of Navicula with 

 costate striae, described in this contribution, in the genus Navicula, because 

 I believe the genera Stauroneis, Ehr., and Pinmdaria, Ehr., to be merely 

 sections of the genus Navicula, and the characters on which they are estab- 

 lished of a purely specific nature. Even the late Prof. Smith did not adhere 

 strictly to the definition of these two genera, as given by Ehrenberg, for we 

 find in the ' Synopsis' that he places in the genus Finyiularia species which 

 have the features of Stauroneis, i.e., F. divergens, P. intemipta, and P. 

 Stauronei-formis. In like manner Finmdaria Johnsoidi, Sm., is a Navicnla 

 in the acceptation of Ehrenberg. 



