170 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



a brief one, so I have judged it well to frame a fuller account from 

 M. Cossmann's description of M. Bernayi, and from specimens of 

 M. ctjtheraformis, Desh., which he has kindly sent me. 



Group characters. — Shell oval, equally rounded at each end, tumid, 

 with prominent umbones, which are inclined forwards, and slightly 

 recurved, so that the front view of the closed valves is cordiform. 

 The lunule is large, and feebly defined by a faint line. Escutcheon 

 not defined. 



The hinge-plate is thick, and the teeth are fairly strong ; there arc 

 three in each valve, divergent and unequal ; the right posterior is 

 curved and strongly bifid throughout its length ; in the left the 

 anterior and median are united at top and do not quite reach the 

 shell-border, and both are entire (not grooved). The anterior part of 

 the hinge-plate forms a small concavity under the lunule. The pedal 

 scar is merged in that of the anterior adductor. The hinge-plate and 

 teeth have, in fact, quite as great a resemblance to that of Venus 

 lamellata as to that of V. exalhida. 



Such being the characters of the type species and of several others, 

 namely, cythercefortnis, Desh., inopinata, Desh., and delicatula, Desh., 

 it becomes a question whether some of the other species referred 

 to Mercimonia by M. Cossmann can remain in it. Thus, two of the 

 species, V. solida, Desh., and V. secunda, Desh., differ in being much 

 more oblique and in having no anterior concavity, the right anterior 

 tooth being very small and placed directly under the lunule. 

 M. fallaciosa, again, is a thin shell with a very small anterior cavity. 



Four other species differ still more widely, for in them the right 

 posterior cardinal is entire (not bifid), and is widely separated from 

 the other teeth, the hinder part of the hinge-plate being excavated 

 and abbreviated. They appear to me to belong to the group which 

 M. Cossmann called Venerella, and I shall refer to them again under 

 that head. 



It will be noticed that in the above description no mention is made 

 of the pallial sinus, the reason being that it is curiously variable ; 

 thus, in the type species there is hardly any sinus, inopinata has 

 a small but distinct triangular sinus, cyther<Bformis has a deeper 

 triangular one, and in fallaciosa it is deep, but horizontal and rounded. 

 It is clear, therefore, that the sinus presents no constant character, 

 and that the siphons of Mercimonia must have varied in length to 

 a remarkable extent. 



16. Chione, Megerle. 



This genus is represented by, a single species in the Oligocene of 

 Pierrefitte in the district of Etampes. It was first figured and 

 described as Venus Loeioyi by Stan. Meunier in 1880,^ and again by 

 Messrs. Cossmann & Lambert in 1884.^ It was recognized as a CJiione 

 by M. Cossmann in 1887, and is the earliest known member of the 

 typical group of this genus in Europe. 



• Nouv. Arch, du Museum, ser. ii, p. 241, pi xiii, figs. 11, 12. 



* Mem. Soc. Geol. de France, ser. iii, vol. iii, p. 79, pi. i, fig. 22. 



