DELTOID. ([Greek: D], _delta_.) Triangular. DENDOSTREA. Sw. ([Greek: Dendron], _dendron_, tree; [Greek: ostreon], _ostreon_, oyster.) Ostrea _Crista-galli_, and other species which are attached to stems of sea-weed and corallines, by means of arms thrown out from the inner surface of the lower valve. Fig. 181, Ostrea Folium. DENTALIUM. Auct. (_Dens_, a tooth.) _Fam._ Maldania, Lam. _Order_, Cirrobranchiata, Bl.--_Descr._ Tubular, arched, increasing in size towards the anterior extremity, open at both ends; small aperture sometimes having a lateral fissure; large aperture round; external surface ribbed, striated or smooth.--_Obs._ The well known shells composing this genus are shaped very much like an elephant's tusk, and are not liable to be confounded with any other genus. The fossil species are sometimes termed Dentalithes, from _dens_, a tooth, and _lithos_, a stone. The Dentalia, being true molluscs, are not rightly placed among the Annelides. Fig. 2, D. octogonum. Found on sandy shores in most climates. 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.