38 DIAPHRAGM. 
continuous, thickened ; operculum horny, composed of numerous 
whorls. Obs. Recent species of this genus are not very nume- 
rous. The fossil species are found in the tertiary deposits. D. 
Jaciniata, fig. 352. 
DELTHY’RIS. Dalman. Fam. Brachiopoda, Lam. Deser. 
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 Spirifer, Sow. Fig. 205, 
D. Plycotes, Dalman. 
DELTOID. (4, delta.) Triangular. 
DENDOS'TRA#A. Sw. (Acvdpov, dendron, tree; osrpeov, ostreon, 
-oyster.) Ostrma Crista-galli, and other species which are attached 
to stems of seaweed and corallines, by means of arms thrown 
out from the outer surface of the lower valve. Fig. 181, Ostreea 
Folium. 
DENTA’LIUM. Lam. (Dens, a tooth.) Fam. Maldania, Lam. 
Order. Cirrobranchiata, Bl. Deser. 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 fossilspecies are sometimes termed Den- 
talithes from dens, a tooth, and lithos,a stone. The Dentalia, 
being true molluscs, are not rightly placed among the Annelides. 
Fig. 2, D. octogonum. 
DEX’TRAL Spiral Shells. Place the point of a spiral shell to- 
wards the eye, with its mouth downwards; if the aperture be on 
the right side of the axis, it is a deatral shell; if otherwise, it is 
sinistral or reversed. 
DEXTRAL Valve. Place the bivalve shell with the posterior 
side (or that on which the ligament is placed) towards the eye, 
with the umbones uppermost; the valves will then be in their 
right position with regard to right and left. j 
DIADE’MA. Ranz. Coronuia Diadema, Auct fig. 17. 
DIAN’CHORA. Sow. Fam. Pectinides, Lam. Order. Pallio- 
branchiata, Bl. Deser. Inequivalve, attached, oblique, sub-trian- 
gular; 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. 
DIA’PHANOUS. (Aca, dia, through; awa, phaino, to shine.) 
Transparent. 
DIAPHRAGM. (d:appaype, diaphragma, a partition.) This 
term is applied to the septa, by which the chambers of multilo- 
cular and other shells are divided from each other. 
