BIVALVIA. 147 



as in some of the species of Lucina, 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. Crao- 



o" 



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. trigonula, 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 Lucina, 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 Hanky, 1849. 



MYSIA. "Leach," Lam., 1818. King, 183- ? S. Wood, 1840. 

 VENUS (sp). Penn. Mont. Flem. Phil. Lovtn. 

 LUCINA (sp.). Turt. Lam. 

 CYTHEREA (sp.) Macgill, 1843. 

 ARTEMIS (sp.). Alder, 1847. Becluz. 

 DOSINIA (sp.). Gray, 1847. 



Generic Character. " Shell more or less orbicular, rather thin, equi valve, 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. 



