Part 32

DENTATED. Having teeth or raised points.

DENTICULATED. (Denticulatus, Lat.) Having little teeth or raised points.

DEPRESSED. Flattened, pressed down, as the spires of some shells.

DEXTRAL Spiral Shells. Place the point of a spiral shell towards the eye, with its mouth downwards; if, as in most instances, the aperture be on the right side of the axis, it is a dextral shell, if otherwise, it is sinistral or reversed. Balea (fig. 296), and Clausilia (fig. 295), are examples of reversed shells.

DEXTRAL Valve. Take a bivalve shell closed, place it before the eye, with the umbones uppermost, and the posterior side, which may be known by the ligament towards the observer, whose right side will then correspond with the right valve of the shell.

DIADEMA. Ranz. CORONULA Diadema, Auct. fig. 17.

DIANCHORA. Sow. Fam. Pectinides, Lam. Order, Palliobranchiata, Bl.--Descr. Inequivalve, attached, oblique, subtriangular; attached valve, having an opening in the place of the umbo; the other valve auriculated, with an obtuse umbo; hinge without teeth.--Obs. The green sand fossils contained in this genus differ from Plagiostoma in being attached. Fig. 175, D. striata.

DIAPHANOUS. ([Greek: Dia], dia, through; [Greek: phainô], phaino, to shine.) Transparent.

DIAPHRAGM, ([Greek: diaphragma], a partition.) This term is applied to the septa, by which the chambers of multilocular and other shells are divided from each other.

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."

DISCOIDAL. (Discus, a circular plane.) A spiral shell is said to be discoidal, when the whorls are so horizontally convolute as to form a flattened spire. Ex. Planorbis, fig. 311. Orbulites Discus, fig. 479.

DISCOLITES. Montf. A genus of microscopic Foraminifera.

DISCONTINUOUS. Interrupted. Ex. The siphon of Nautilus is discontinuous, i. e. its termination in one chamber does not reach to its commencement in the next. The varices of Triton, occurring in different parts of the whorls, do not form the continuous ridges which characterize the generality of the Ranellæ.

DISCORBITES. Lam. A genus of microscopic Foraminifera.

DISTANT. The teeth on the hinge of a bivalve shell are said to be distant when they are remote from the umbones.

DIVARICATED. Diverging, meeting in a point, as the teeth on the hinge of Placuna, fig. 184.

DOLABELLA. Lam. (Dim. from Dolabra, a hatchet.) Fam. Aplysiacea, Lam. and Bl.--Descr. Hatchet-shaped, arched, covered with a horny epidermis; posteriorly attenuated, thickened, sub-spiral, anteriorly plane, broad, thin; posterior margin reflected.--Obs. The two or three species of Dolabella known are inhabitants of the Indian Ocean. They were placed by Linnæus in his very convenient genus Bulla, under the name B. dubia. Fig. 255, Dolabella Rumphii.