(Pl. I. II. IV. V.); or, the cuirass wanting, the epidermis is naked (Pl. III. et VI.).
The cuirass, answering, in the greatest number of animalcules, to the valve of muscles, is composed of two siliceous and transparent parts, surrounding the animal. The univalve cuirass is a leaf rolled on itself (Fig. 5—17., 38—17), the edges of which are confounded together, so that they never form, except a few small openings, a valve surrounding entirely the animalcule. That cuirass appears smooth, for instance, in the Frustulia appendiculata. Ag. (Pl. I. fig. 12—13. a.), or striped, as in the Navicula costata (Pl. I. fig. 9—11.), where the stripes are distributed like rays, setting from an ombilic, to be found in one of the middle ribs of the cuirass (fig. 9. a.); or merely ribbed; or provided with strong and elevated ribs, such as in the Navicula ciliata (Pl. I. fig. 5. a.), where the ribs are besides covered with fine hair.
Towards the lower parts, the cuirass forms in the Naviculae and Frustuliae a flat surface, generally quadrangular (parallelogram), called pedal surface (fig. 8. e., fig. 13. b.), to which two animalcules are always adhering.
In the Diatomeae, for ex, in the Diatoma fenestratum (Pl. IV. fig. 38.), the cuirass is flattened on the edges, and cut in half circles, whilst, where the animalcules are adhering, it forms a narrow surface of reunion (fig. 38. b.). The form of this surface corresponds always with that of the cuirass, viz. it
