BIVALVIA. 147 
as in some of the species of Zwcina, showing the muscle marks deeply impressed ; 
they are nearly equal in size, though the anterior one is rather narrower, the line 
of the mantle is without the least inflection: in the thickening of the interior a 
ridge is produced near the upper anterior margin, giving the appearance of an addi- 
tional muscle mark; a similar appearance may be seen in the old specimens from 
the Coralline Crag, besides an obscure ridge running down the centre dividing it 
into two nearly equal parts. There is no doubt of this species being identical with 
the Belgian fossil, and according to M. Nyst, it is said to have been found in the 
Older Tertiaries from the environs of Paris. I have not seen it from the Mam. Crag. 
D. apicalis, Phil., somewhat resembles the young of our shell, but it appears to 
have a greater comparative height from the umbo to the margin; and I have been 
unable to see a specimen of this or of D. ¢rigonula, Bronn, which also does not very 
greatly differ. 
The dentition of this species precisely resembles that of D. rotundata, as well as 
the muscle marks of the interior, from which it is presumed to belong to the same 
genus, but the interior of aged specimens is thickened like those of Zwcina, to 
which it appears to be very closely related. The specimen figured has a somewhat 
sinuated form in the margin on the posterior side, which is merely accidental; it was 
selected for the purpose of showing the interior. 
Lucinopsis, Forbes and Hanley, 1849. 
Mysta. ‘ Leach,’ Lam.,1818. King, 183-? S. Wood, 1840. 
Venus (sp). Penn. Mont. Flem. Phil. Lovén. 
Lucrna (sp.). Zurt. Lam. 
Cyrerea (sp.) Macgill, 1843. 
ARTEMIS (sp.). Alder, 1847. Reeluz. 
Dostnta (sp.). Gray, 1847. 
Generie Character. “Shell more or less orbicular, rather thin, equivalve, slightly 
inequilateral, closed; surface smooth or concentrically striated, inner margin entire ; 
muscular impressions oblong or suborbicular, nearly equal. Palleal sinus wide, deep, 
central, obtuse. Hinge composed of two diverging central teeth, one of which is 
bifid in the right valve, and three, the central one bifid, in the left. Ligament 
external, prominent, rather long. No defined lunule.”’ 
“ Animal suborbicular, its mantle freely open, the margins entire. Siphonal tubes 
short, diverging, separate, the branchial with its orifice fringed, the anal simple. 
Foot lanceolate. Labial paps, small, triangular.” 
This being the first and only diagnosis of the genus I have seen, it is copied from 
the authors of the ‘ Hist. of British Mollusca,’ whose name is here adopted. 
The long and well-known British species Venus undata, of Pennant, is considered 
as the type of the genus. 
