Mr. T. U. Jones on some species of Lcperditia. 93 



the ventral margin as a projecting edge*, keeping the vertical 

 direction of the valve, instead of being a mere moulding accom- 

 panying an incurvation of the ventral border. Further, in 

 fig. 10 c {op. cit.), tiiere is represented an inverted edge to the 

 right t valve (as in the left valve of L. Baltliica) ; and Count 

 Keyserling remarks, that, whilst the ventral plate in L. Balthica 

 is marked with transverse stria?, in this species it is smooth. 



The Petschora specimens vary from about ^^^p to ^| inch, and 

 even reach nearly 1^ inch, in length. 



The anterior and the central tubercle are both referred to by 

 M. Keyserling as being well marked in L. marg^xnata, and the 

 cast of the inside of the central tubercle is described as being 

 beset with crowded unequal wart-hke markings, and accompanied 

 by ill-defined, tortuous, fine vein-markings, radiating backwards. 

 The sulcus defining the marginal ledge is stated to be " slight 

 on the surface of the valves, and deepened on the cast.'' 



M. EichwaldJ describes and figures a form under the deno- 

 mination of " Cypridina Balthica," and at the same time recog- 

 nizes in the casts, at least, the " margins " seen in Keyserling's 

 species, and apparently allows the latter species to be well 

 established on that character. The individuals of M. Eichwald's 

 species vary in length from ^ to |^ inch (according to his figures); 

 and in the text he gives " ^ inch " as the size. They possess the 

 anterior and central tubercles, with the muscular impression and 

 radiating vascular markings. Excepting that the outside, ac- 

 cording to the author, exhibits no marginal sulcus and rim, 

 though the internal east does, M. Eichwald's figures present no 

 material differences from the smaller form of M. Keyserling's 

 species. Neither the figures, however, nor the description, 

 aflFord all the necessary details for satisfactory comparison with 

 known species. 



The specimens here referred to are from " the upper beds of 

 thegreywacke limestone" of Esthland and Livonia; and, accord- 

 ing to M. Eichwald {op. cit.), another similar form, but with a row 

 of punctiform pits on the border of the valve [query, on a de- 

 pressed marginal border?], and without any apparent central 

 spot and radiating vessels, occurs in the dolomitic limestone 

 near Gatschina, on the river Oredesch§. 



* It is described as " more or less clearly running along all the oval out- 

 line of the valve and ending at the hinge-angles." 



t As this inversion on the right valve is contrary to what obtains in 

 other species of the genus, perhaps this figure has been reversed on the 

 plate. 



X Bulletin Imp. Soc. Nat. Moscou, 1854, part I. p. 99. pi. 2. figs. / & 8. 



§ M. Eichwald also figures and describes a naiTower and slightlv arched 

 form (" Cypridina minuta," loc. cit. fig. 6), from the Brandschiefer"of Erras 

 and the vicinitv of Talkhof. 



