. Fresh-water rhizopods of North America [microform]. Rhizopoda; Fresh-water fauna; Rhizopodes; Faune d'eau douce. GENUS NEBELA—NEBELA COLLARIS. 151 In all other respects, the forms agree vitli the varieties of N. collaris, but I have never met with specimens in which the shell was otherwise than com- pressed. Mr. Carter, in the same work, described a form under the name of Difflngia peltigcmcea, which probably also belongs to the same animal as Nebcia collaris. The nature of the singularly varied shell of Nehela collaris I have not been able to determine with any satisfaction. In the characte
. Fresh-water rhizopods of North America [microform]. Rhizopoda; Fresh-water fauna; Rhizopodes; Faune d'eau douce. GENUS NEBELA—NEBELA COLLARIS. 151 In all other respects, the forms agree vitli the varieties of N. collaris, but I have never met with specimens in which the shell was otherwise than com- pressed. Mr. Carter, in the same work, described a form under the name of Difflngia peltigcmcea, which probably also belongs to the same animal as Nebcia collaris. The nature of the singularly varied shell of Nehela collaris I have not been able to determine with any satisfaction. In the characteristic forms, the elements of structure, the disks and plates, appear to be intrinsic, and not of a foreign character. They appear to be cemented together or conjoined at the borders, and not implanted upon or incorporated witli a distinct chitinoid membrane. In breaking the shell, the line of rupture follows the outlines or intervals of the disks and plates. The shell ajjpears to be silicious, as it remains unchanged when exposed to +he action of heated sulphuric and nitric acids. Dr. Wrllich, in referring to the structure of the shell of the transitional forms of Difflngia symmetrica, which, as previously intimated, 1 suspect to belong to Nehela collaris, calls the peculiar elements colloid disks and plates. He remarks of them that they are derived from the animal, and not directly from the medium in which it lives. He supposes, however, that they are formed through the coalescence of diatoms and other mineral elements with the chitinoid basal substance of the shell, which then undergo metamorphosis into all the colloid forms that occur.* Of this process I have been unable to satisfy myself; but the exceed- ingly varied specimens which have come under my notice, of shells composed of elements appar- ently intrinsic and of regular bu+ widely different forms, of others apparently of extrinsic 'ements regular and irregular, witli many (Others of a transi- tional character, wou
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