EBURNA. 



141 



by the ligamentary pit in the hinge. Sandy shores in all climates. 

 Fig. 108, D. cuneatus. 



DORSAL. A dorsal shell is one placed upon the back of the ani- 

 mal. The dorsal margin of a bivalve shell is that on which the 

 hinge is placed ; the opposite margins are termed ventral. The 

 dorsal surface of a spiral univalve is that which is seen when the 

 aperture is turned from the observer. The dorsal valve is the 

 uppermost in Brachiopodous bivalves. The dorsal part of a 

 symmetrical convolute univalve, such as the Nautilus and Am- 

 monite is that part of the whorls which is at the greatest dis- 

 tance from the spire, that is, the outer part of the whorls Thus 

 the situation of the siphon is said to be dorsal when it pierces the 

 septum near the outer edge of the whorls, The dorsal part of 

 symmetrical conical univalves, such as Patella, is the upper part, 

 on which the apex is placed. 



DORSALIA. Lam. (Dorsum, the back ) The first family of the 

 order Annelides Sedentaria, Lam. containing the genera Areni- 

 cola, not a shell, and Siliquaria, fig. 1, which is now considered 

 as a true mollusc, and placed next to Vermetus. 



DOSINA. Schum. Venus Verrucosa, Casina, and similar spe- 

 cies. Fig. 119, a. 



DREISSINA. Mytilus Polymorphs. Auct. fig. 159. This 

 genus differs from Mytilus principally in the characters of the 

 animal. The shell is characterized by a small septiform plate 

 under the hinge within. Fluviatile, Europe and Africa. 



EBURNA. Lam. (Eburneus, ivory.) Fam. Purpurifera, Lam. 

 Entomostomata, Bl. — Descr. Oval, thick, smooth, turrited, um- 

 bilicated ; spire angulated, acute, nearly as long as the aperture ; 

 aperture oval, terminating anteriorly in a canal, posteriorly in a 

 groove ; outer lip slightly thickened with an anterior notch, 

 which terminates in a spiral fold surrounding the body whorl ; 

 umbilicus generally covered by the thickened columellar lip. — 

 Obs. The beautiful shells called ivory shells, which originally 

 constituted part of this genus, are now placed in the genus An- 

 cillaria by authors. They differ from the present genus Eburna, 

 in having the sutures of the spire covered with a polished enamel. 



