DOLABELLA. 39 
DI'CERAS. Lam. (Ass, dis, double; Kepas, ceras, horn.) Fam. 
Chamacea, Bl. and Lam. JDescr. Inequilateral, inequivalve, 
attached by the point of the umbo of the larger valve; um- 
bones prominent, spirally twisted and grooved; hinge with one 
large thick tooth, inthe 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 dis- 
tinguish it from any other, although it appears to approach 
Isocardia in some characters. The singular fossil shells compo- 
sing this genus are found in granular limestone, near Geneva 
and in Normandy, Fig. 154, D. perversum. 
DIDON’TA. Schum. Saxicava, Auct. 
DIGITATED. (Digitus, finger.) Branched out in long points, 
as Ricinula, fig. 413. 
DIMORPHINA. D’Orb. A genus of microscopic Foraminifera. 
DIMYA'RIA. (Ass, dis, double; pvov, myon, muscle.) The first 
order of Conchifera, Lam. including those molluscs which have 
two adductor muscles. The Conchifera Dimyaria are divided 
into Crassipedes, Tenuipedes, Lammellipedes, and Ambigue. 
Fig. 44 to 155. 
DIOICA. Bl. The first division of the class Paracephalophora, 
Bl. Divided into the orders Siphonobranchiata and Asiphoni- 
branchiata, Bl. 
DI'PLODON. Spix. Hyrra Syrmatophora, Lam. fig. 144, and 
Unio multistriatus, Lea, are doubtfully quoted by Lea as belong- 
ing to this apparently ill defined genus of Nayades. 
DIP’SAS. Leach. A genus or sub-genus of Nayades, the 
distinctive character of which, is “ having a linear tooth under 
the dorsal edge.’”’ D. plicatus, fig. 142. 
DISCINA. Lam. Oreicuna, Auct. 
DISCOI'DAL. (Discus, a circular plane.) A spiral shell is said 
to be Discoidal, when the whorls are so horizontally convolute 
as to form a flattened spire. Ez. Planorbis, fig. 311. Orbulites 
Discus, fig. 479.* 
DIS'COLITES. Montf. A genus of microscopic Foraminifera. 
DISCONTINUOUS. (Interrupted.) Ez. The siphon of Nauti- 
lus is discontinuous, i.e. its termination in one chamber does 
not reach to its commencement in the next. The varices of 
Triton, occurring in different parts of the whorls, do not form 
the continuous ridges which characterize the Ranelle. 
DISCORBI'TES. Lam. A genusof microscopic Foraminifera. 
DISTANT. The teeth on the hinge of a bivalve shell are said to 
distant when they are remote from the umbones. 
DOLABEL'LA. Lam. (Dim. from Dolabra,a hatchet.) Fam. 
Aplysiacea, Lam. and BI. Deser. Hatchet-shaped, arcuate, co- 
vered with a horny epidermis: posteriorly narrowed, thick- 
