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 is narrow in the flat cuirass, and triangular in the triangular cuirass of the Desmidium Swartzii (Pl. IV, fig. 44.). The form of the bivalve cuirass is always analogous to that of the animalcule. Surirellae (for ex. the Surirella Venus (fig. 1—4.) have, according to the length of the animalcule, an open cuirass, each valve of which is like a pot, the one included in the other, in a very peculiar manner, forming a sort of inge (fig. 4. b.), nearly similar to a round snuff-box. Their edges, besides, are supplied with rounded and radiated elevations (fig. 4. A.), between which are seen furrows of the same size (fig. 4. B.). Though different, the bivalve cuirass of the Closteria has an analogous form. Each valve (fig. 61. A. A.) forms cornucopiae, the uniting point of which is seen in the middle of the animalcule, where the sexual openings (fig. 61, c.) are to be found. No inge is to be seen, but the two thin edges of the valves (fig. 62. d.) are placed immediately one upon the other. This cuirass is open at each point of a valvule, its being the place where we see the opening which I consider as the mouth (fig. 61. 63. b. b. 64. 65. a. a.)

In the Pleurosicyos myriopodus, the cuirass has two valves, of the same form as in the Closteria. It is, besides, octangular (fig. 69.), and on each side of an angle, 8—12 pedal holes (fig. 68. b.) are seen.

The Cosmaria must be considered as double animals, forming together a double cuirass, and each separately a single one.