138 DIFFUSE. 



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, Pallio- 

 branchiata, Bl. — Bescr. Inequivalve, attached, oblique, sub- 

 triangular ; 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. (Am, dia, through ; <pairo), phaino, to shine.) 

 Transparent. 



DIAPHRAGM. (Eiacppayfia, 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. (Ave, dis, double ; Kepag, ceras, horn.) 

 Fam. Chamacea, Bl. and Lam. — Bescr. Inequilateral, inequi- 

 valve, 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 consi- 

 dered as Chamse 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. (Difundo, 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 be- 



