DIATOMACE.E. 79 



The markings consist of a row of oblong dots or 

 puncta (punctum, a point) . This species often forms 

 yellowish layers upon damp paths, &c. 



In the next group, the valves have a longitudinal 

 line running down the middle of the valves, with a 

 little knob or nodule in its centre (PL V. fig. 22 s), 

 both consisting of internal thickened portions of the 

 valves. 



In Cocconeis placen'tula (Pl.V. fig. 33) the valves are 

 oval, and the markings consist of longitudinal rows 

 of minute dots, with a marginal row of puncta ; these 

 markings are invisible under ordinary illumination. 



InGomphonema acumindtum (Pl.V. fig. 17) thefrus- 

 tules are attached to a branched stalk (stipes) ; they are 

 wedge-shaped or ciineate (cuneus, a wedge) and trans- 

 versely striate (fig. 18), the striae consisting of dots. 



In Pinnuldria vir'idis (PL V. fig. 22), which is very 

 often seen slowly traversing the field of the micro- 

 scope when a drop of pond-water is examined, the 

 frustules in the front view are linear, the valves being 

 elliptic oblong, and transversely striated, the striae 

 consisting of furrows. 



In Gyrosig'ma (Pleurosig'ma) attenudtum (PL VI. 

 fig. 16) the valves are sigmoid, or somewhat resemble 

 a Greek 9 (sigma) in outline, and the markings con- 

 sist of rectangularly crossed rows of very fine dots ; 

 in the front view, the frustules are linear-oblong with 

 truncate ends. 



Tabelldria flocculosa (PL VI. fig. 23) has the frus- 

 tules adherent only at the angles, as in Didtoma. They 

 are rectangular, and in the front view exhibit a row 

 of longitudinal dark lines interrupted in the middle ; 

 these have been compared to the vittse of the fruit of 

 the Umbelliferae, and have received the same name. 



Among the marine species may be mentioned 

 Melosira nummuloides (PL VI. fig. 9), in which the 

 frustules are united into a chain or cylindrical fila- 

 ment. This is very common among sea- weeds, &c, ; 



