8 GREVILLE, ON DTATOMACEiE. 



A. lineata, Greg.^ Podocystis Americana, Bail., Climacosphenia 

 moniligera, Eh., Rhabdonema arcuatum and R. Adi'iaticum, 

 Kiitz., Grammatophora hamulifera, Kiitz., Asterolampra impar, 

 Shadb. 



1. Cocconeis punctatissima, Grev. Valve elliptical-oA^al, 

 densely areolato-punctate ; striae moniliform, concentric with 

 the extremities, the moniliform structure ceasing within the 

 margin so as to leave a simply striated border ; median line 

 dilated towards the extremities. Length of frustnle 0'0020" 

 to 0-0024" ; breadth 00012" to 0-0015". Strice 20 in 0-001". 

 (PL III, fig. 1.) 



Hah. — All the species in this paper are marine, from the 

 Island of Trinidad. 



One of the most beautiful species I am acquainted with, 

 and closely allied to a new MS. species from the Black Sea, 

 distributed by Professor Smith under the name of Cocconeis 

 Momsii ; in fact, a mere cursory examination might readily 

 leave an impression on the mind that the two were nothing 

 more than varieties. A careful comparison, however, has 

 satisfied me that they are truly distinct. The general out- 

 line is the same in both, but the strise in C Morrisii are 

 coarser and far less numerous than in the West Indian Diatom. 

 The beading is also much largei', more equal in size, and 

 more separated. In C. punctatissima the beading, in con- 

 sequence of the very numerous striae, becomes more minute 

 as it joins the median line, which latter presents the apj)ear- 

 ance of a rather broadly linear, indistinctly defined, pale space, 

 dilated towards the extremities, Avhere it terminates elliptically. 

 In the single valves, which are most fi-equently met with, there 

 is but one side, as it were, of tlie median line visible, which 

 gives them a very peculiar aspect. 



It is more difficult to separate this species by a written 

 character from the variable C. placentula, although the 



who defines the striae as simply costate. Under these circumstances, I take 

 tlie opportunity of offering an illustration of A^. crabro, as it occurs in my 

 Trinidad gathering, in the liope that it may be regarded as a faitliful typical 

 represeutalion. Tiie truth is, the group to which this species of Navicula 

 belongs is one of great perplexity, and I trust that my friend, Professor 

 Gregory, who is studying it, will do something towards clearing it up. The 

 two forms which lie has named iV. nltida, Sm. ? in the fourth volume of the 

 ' Transactions of the Microscopical Society,' will probably prove to be the 

 same as our present species, the moniliform structure having been perhaps 

 overlooked from its extreme obscurity. As the name above mentioned does 

 not occur in tlic second volume of" Professor Smith's 'Synopsis,' that of 

 N. crabro has doubtless been substituted for it. The outline and general 

 character of Professor Gregory's N. ? pandura, Breb. ? is also extremely 

 similar to the Trinidad Diatom. 



