DICERAS. Lam. ([Greek: Dis], _dis_, double; [Greek: Keras], _ceras_, horn.) _Fam._ Chamacea, Bl. and Lam.--_Descr._ Inequilateral, inequivalve, attached by the point of the umbo of the larger valve; umbones prominent, spirally twisted and grooved; hinge with one large thick tooth in the larger valve; muscular impressions, two in each valve.--_Obs._ The prominent spiral umbones, which give rise to the name of this genus, with the circumstance of its being attached by the point of one of them, is sufficient to distinguish it from any other, although it appears to approach Isocardia in some characters. In others it will be found still more nearly to resemble Chama. In fact, from being attached and irregular, the shells composing this genus have been considered as Chamæ with produced umbones. The singular fossil shells composing this genus, are found in granular limestone, near Geneva and in Normandy. Fig. 154, D. perversum. DIDONTA. Schum. SAXICAVA. Auct. DIFFUSE. (_Diffundo_, to spread out, to dilate.) A term applied to the aperture of a univalve shell, when it is spread out or widened into a flat surface, or digitations. _Alated_ is another term used to express the same character. Thus, the shells belonging to the family of Alatæ, in the system of Lamarck, are _diffuse_ in the outer lip. Fig. 402 to 406. DIGITATED. (_Digitus_, finger.) Branched out in long points, as Ricinula, fig. 413. DILATED. Expanded, spread. This term has the same application as diffuse and alated, explained above. The outer lip of Rostellaria Columbaria, fig. 403 (Hippochrenes, Montf.), will serve as an example. DIMORPHINA. D'Orb. A genus of microscopic Foraminifera. DIMYARIA. ([Greek: Dis], _dis_, double; [Greek: muon], _myon_, muscle.) The first order of Conchifera, Lam. including those molluscs which have two adductor muscles, and consequently two muscular impressions in each valve. The Conchifera Dimyaria are divided into Crassipedes, Tenuipedes, Lamellipedes, and Ambiguæ, fig. 44 to 155. DIOICA. Bl. The first division of the class Paracephalophora, Bl. It is divided into the orders Siphonobranchiata and Asiphonibranchiata, Bl. DIPLODON. Spix. HYRIA Syrmatophora, Lam. fig. 144, and UNIO multistriatus, Lea, are doubtfully quoted by Lea as belonging to this apparently ill-defined genus of Nayades. DIPSAS. Leach. A genus or sub-genus of Nayades, the distinctive character of which is "having a linear tooth under the dorsal edge." D. plicatus, fig. 142. DISCINA. Lam. ORBICULA, Auct. DISCODOMA. Sw. A sub-genus of Lucerninæ, Sw. (Helix), thus described, "teeth none; aperture angulated; the inner lip nearly obsolete; the outer only slightly thickened; margin carinated."