92 Mr. T. R. Jones on some species of Leperditia. 



serl/' The carapaces have mostly disappeared; but some re- 

 main in an altered crystalline state, or as a pulverulent, white, 

 calcareous substance. The casts of the interior, and sometimes 

 of the exterior, of the valves are very distinct ; but the saccharoid 

 crystalline character of the matrix has been ill adapted to receive 

 the impression of any fine vascular or other markings, if such 

 existed in the originals. 



Casts of a right and a left valve are figured in PL VII. figs. 11, 

 12; and, having been able, in some instances, to observe portions 

 of the carapace in place, as vi^ell as good casts of the outside, I 

 am satisfied that the exteriors of the valves presented almost, if 

 not quite, as strong a marginal rim as is seen in the casts. This I 

 especially mention, because M. Eichwald, in describing* speci- 

 mens of a similar form (possibly identical), from Esthland and 

 Livonia, states that the outsides of the valve have no marginal 

 sulcus and rim, and that the casts alone exhibit this character. 



The St. Petersburgh specimens present the following cha- 

 racters : — 



(inch) (inch) 

 Dimensions : — Length \ to -^-^^ 

 Breadth ^ to /^ 



Carapace- valves almost equivalve, nearly oblong (subject to 

 slight variation in outline), broadest at the posterior third, 

 straight at the back, obliquely curved on the ventral margin, and 

 obliquely rounded at the extremities ; the convexity of the sur- 

 face is nearly central. The central portion of the ventral edge 

 of each valve is somewhat incurved ; that of the right valve ap- 

 parently overlapping that of the left. 



The right valve is margined, except on the dorsal edge, by a 

 well-marked sulcus, running along at a short distance from the 

 edge, and separating off a narrow, flattened, or slightly convex 

 border. On the left valve there is a similar marginal rim, 

 but it dies out on the central incurved portion of the ventral 

 margin, fig. 12 6. 



The anterior tubercle is distinct; the central tubercle can 

 also be easily discerned on the casts ; the radiating vascular mark- 

 ings, however, are not apparent. 



Of M. Keyserling's figures of Leperditia marginata [op. cit.), 

 fig. 16 ^ (the smallest figure) alone corresponds exactly with the 

 form under notice ; in fig. 16^ {op. cit.), and apparently in fig. 16a, 

 the marginal rim (of right valve) is indicated as passing round 



* Moscow Bulletin, 1854. 



