DEXTRAL. 137 



ened ; operculum horny, composed of numerous whorls.— Obs. 

 Several fossil species are found in the tertiary deposits. D. 

 laciniata, fig. 352. Recent species belong to tropical climates. 

 DELTHYRIS, Dalman. Fam. Brachiopoda, Lam.— Descr. Hinge 

 more or less rounded, with distant umbones ; both valves convex ; 

 with the umbo of the largest rostrated and deltoid, with a hollow. 

 This genus forms part of the genus Spirifer, Sow. Fig. 205. D. 

 Plycotes, Dalman. 

 DELTOID. (A, delta.) Triangular. 



DENDOSTREA. Sw. (AevSpov, dendron, tree ; oarpeoy, 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. {Bens, 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 



