88 REV. E. o'mEARA, 



Plar/iotr'opidea. 



This embraces only one genns, Plagiotropis , gen. nov. 

 The development of the median lines into prominent keels, 

 which in the preceding group occurred to a slight extent, is 

 more strongly marked in the two to be next treated, in 

 which the six nodules appear again in the normal manner. 

 The only species to be here assigned Avhieh occurred to the 

 author in a living state was found in brackish water in the 

 harbour of Pillau, and is distinguished from the next related 

 genus, Amphiprora (Ehr.), by the position of the keel, 

 which, instead of being central, is strongly excentric ; and 

 also by the disappearance of the prominent longitudinal 

 striae, which along with the same occur in all the Amphi- 

 prorea. The valve of P. baltica is sharply lanceolate in 

 outline, resembling Navicula serians (Kiitz.), in breadth 

 from one-fifth to one-sixth of its length ; the keel describing 

 a gentle curve, not sigmoid, but, as in the AtnphiproreoB , 

 sinking down at the central nodule to the plane of the valve 

 which it divides into two parts in the proportion of one to 

 foui*, so that it is very excentric. Supposing the Plagio- 

 tropis to lie so as to present its valve-surface to the observer, 

 on the upper valve the keel deviates towards the right, on 

 the under-valve towards the left, so that Plagiotropis, like 

 Pinnularia, is diagonally constructed. The valves exhibit a 

 very fine striation, and when dry are nearly colourless. 

 The girdle-band view strongly resembles that of an Amphi- 

 prora, pretty much that of Amphiprora indica (Grun.), only 

 that the two keels obviously lie in different planes. The 

 inner structure is similar to that of Navicula. Two endo- 

 chrome-plates lie upon the girdle-bands, and thence with 

 their edges stretch to some extent over the valves. Each, 

 plate covers the greater part of the valve from the keel of 

 which it has extended, the opposite margin going a shorter 

 distance towards the other keel. The structure of the soft 

 parts corresponds with the diagonal construction of the cell- 

 cover. 



AmphiiropidecB. 



In this group we have only a single form — Amphitropis 

 paludosa (Rab.), quere Amphicampa paludosa, E,ab. Fl. Eur. 

 Alg., p. 257. The Amphitropidece , says Pfitzer, are related 

 to the Plagiotropidece somewhat as the Cymbelle(B, still sym- 

 metrical in outline, are to the Navicule<s. The form of the 

 cell-cover differs little, but the inner structure is quite 

 different. The Amphitropis paludosa ( W. Sm.) Rab., is dis- 

 tinguished by means of its sigmoid keels, constructed \i\ 



